Display device

ABSTRACT

To suppress fluctuation in the threshold voltage of a transistor, to reduce the number of connections of a display panel and a driver IC, to achieve reduction in power consumption of a display device, and to achieve increase in size and high definition of the display device. A gate electrode of a transistor which easily deteriorates is connected to a wiring to which a high potential is supplied through a first switching transistor and a wiring to which a low potential is supplied through a second switching transistor; a clock signal is input to a gate electrode of the first switching transistor; and an inverted clock signal is input to a gate electrode of the second switching transistor. Thus, the high potential and the low potential are alternately applied to the gate electrode of the transistor which easily deteriorates.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/825,147, Nov. 29, 2017, now U.S. Pat. No. 10,297,618, which is acontinuation of U.S. application Ser. No. 15/391,938, filed Dec. 28,2016, now U.S. Pat. No. 9,842,861, which is a continuation of U.S.application Ser. No. 14/548,365, filed Nov. 20, 2014, now U.S. Pat. No.9,536,903, which is a continuation of U.S. application Ser. No.13/289,084, filed Nov. 4, 2011, now U.S. Pat. No. 8,902,146, which is acontinuation of U.S. application Ser. No. 11/853,090, filed Sep. 11,2007, now U.S. Pat. No. 8,054,279, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2006-270016 on Sep.29, 2006, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a display device including a circuitformed by using a transistor. In particular, the present inventionrelates to a display device using an electro-optical element such as aliquid crystal element, a light-emitting element, or the like as adisplay medium, and an operating method thereof.

2. Description of the Related Art

In recent years, with the increase of large display devices such asliquid crystal televisions, display devices have been activelydeveloped. In particular, a technique for forming a pixel circuit and adriver circuit including a shift register or the like (hereinafter alsoreferred to as an internal circuit) over the same insulating substrateby using transistors formed of a non-crystalline semiconductor(hereinafter also referred to as amorphous silicon) has been activelydeveloped, because the technique greatly contributes to low powerconsumption and low cost. The internal circuit formed over theinsulating substrate is connected to a controller IC or the like(hereinafter also referred to as an external circuit) through an FPC orthe like, and its operation is controlled.

A shift register which is formed by using transistors formed of anon-crystalline semiconductor (hereinafter also referred to as amorphoustransistors) has been devised among the above-described internalcircuits. FIG. 30A shows a structure of a flip-flop included in aconventional shift register (see Reference 1: Japanese Published PatentApplication No. 2004-157508). The flip-flop in FIG. 30A includes atransistor 11, a transistor 12, a transistor 13, a transistor 14, atransistor 15, a transistor 16, and a transistor 17, and is connected toa signal line 21, a signal line 22, a wiring 23, a signal line 24, apower supply line 25, and a power supply line 26. A start signal, areset signal, a clock signal, a power supply potential VDD, and a powersupply potential VSS are input to the signal line 21, the signal line22, the signal line 24, the power supply line 25, and the power supplyline 26, respectively. An operating period of the flip-flop in FIG. 30Ais divided into a set period, a selection period, a reset period, and anon-selection period as shown in a timing chart in FIG. 30B, and most ofthe operating period is the non-selection period.

Here, the transistor 12 and the transistor 16 are on in thenon-selection period. Thus, since amorphous silicon is used for asemiconductor layer of each of the transistor 12 and the transistor 16,fluctuation in the threshold voltage (Vth) caused by deterioration orthe like occurs. More specifically, the threshold voltage rises. Thatis, since each of the transistor 12 and the transistor 16 cannot beturned on because of rise in the threshold voltage, VSS cannot besupplied to a node 41 and the wiring 23 and the conventional shiftregister malfunctions.

In order to solve this problem, a shift register in which a thresholdvoltage shift of the transistor 12 can be suppressed has been devised inReference 2 (Soo Young Yoon, et al., “Highly Stable Integrated GateDriver Circuit using a-Si TFT with Dual Pull-down Structure”, SOCIETYFOR INFORMATION DISPLAY 2005 INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICALPAPERS, Volume XXXVI, pp. 348 to 351), Reference 3 (Binn Kim, et al.,“a-Si Gate Driver Integration with Time Shared Data Driving”,Proceedings of The 12th International Display Workshops in conjunctionwith Asia Display 2005, pp. 1073 to 1076), and Reference 4 (Mindoo Chun,et al., “Integrated Gate Driver Using Highly Stable a-Si TFT's”,Proceedings of The 12th International Display Workshops in conjunctionwith Asia Display 2005, pp. 1077 to 1080). In Reference 2, Reference 3,and Reference 4, a new transistor (described as a first transistor) isprovided in parallel to the transistor 12 (described as a secondtransistor), and a threshold voltage shift of each of the firsttransistor and the second transistor is suppressed by inputting invertedsignals to a gate electrode of the first transistor and a gate electrodeof the second transistor in the non-selection period.

In addition, a shift register in which not only the threshold voltageshift of the transistor 12 but also a threshold voltage shift of thetransistor 16 can be suppressed has been devised in Reference 5(Chun-Ching, et al., “Integrated Gate Driver Circuit Using a-Si TFT”,Proceedings of The 12th International Display Workshops in conjunctionwith Asia Display 2005, pp. 1023 to 1026). In Reference 5, a newtransistor (described as a first transistor) is provided in parallel tothe transistor 12 (described as a second transistor), and a newtransistor (described as a third transistor) is provided in parallel tothe transistor 16 (described as a fourth transistor). Then, a thresholdvoltage shift of each of the first transistor, the second transistor,the third transistor, and the fourth transistor is suppressed byinputting a signal to a gate electrode of the first transistor and aninverted signal to a gate electrode of the second transistor, andinputting a signal to a gate electrode of the third transistor and aninverted signal to a gate electrode of the fourth transistor in thenon-selection period.

Further, the threshold voltage shift of the transistor 12 is suppressedby applying an AC pulse to the gate electrode of the transistor 12 inReference 6 (Young Ho Jang, et al., “A-Si TFT Integrated Gate Driverwith AC-Driven Single Pull-down Structure”, SOCIETY FOR INFORMATIONDISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS, VolumeXXXVII, pp. 208 to 211).

Note that each of display devices in Reference 7 (Jin Young Choi, etal., “A Compact and Cost-efficient TFT-LCD through the Triple-Gate PixelStructure”, SOCIETY FOR INFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUMDIGEST OF TECHNICAL PAPERS, Volume XXXVII, pp. 274 to 276) and Reference8 (Yong Soon Lee, et al., “Advanced TFT-LCD Data Line Reduction Method”,SOCIETY FOR INFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OFTECHNICAL PAPERS, Volume XXXVII, pp. 1083 to 1086), the number of signallines is reduced to one-third by using a shift register formed using anamorphous silicon transistor as a scan line driver circuit and inputtinga video signal to each of subpixels of R, G, and B from one signal line.In each of the display devices in Reference 7 and Reference 8, thenumber of connections of a display panel and a driver IC is reduced.

SUMMARY OF THE INVENTION

According to a conventional technique, a threshold voltage shift of atransistor is suppressed by applying an AC pulse to a gate electrode ofthe transistor which easily deteriorates. However, in the case whereamorphous silicon is used for a semiconductor layer of the transistor,naturally, it becomes a problem in that a threshold voltage shift of atransistor which forms a circuit generating the AC pulse occurs.

In addition, although it has been proposed that the number ofconnections of a display panel and a driver IC is reduced by reducingthe number of signal lines to one-third (see Reference 7 and Reference8), further reduction in the number of connections to a driver IC hasbeen practically required.

That is, as problems which are not solved by the conventional technique,a problem of a circuit technique for controlling fluctuation in thethreshold voltage of a transistor, a problem of a technique for reducingthe number of connections of a driver IC mounted on a display panel, aproblem of reduction in power consumption of a display device, and aproblem of increase in size and high definition of a display device havebeen left.

It is an object of the present invention disclosed in this specificationto provide an industrially beneficial technique by solving one or aplurality of the aforementioned problems.

In a display device in accordance with the present invention, athreshold voltage shift of a transistor can be suppressed by alternatelyapplying a positive power source and a negative power source to a gateelectrode of the transistor which easily deteriorates.

In addition, in a display device in accordance with the presentinvention, a threshold voltage shift of a transistor can be suppressedby alternately applying a high potential (VDD) and a low potential (VSS)to a gate electrode of the transistor which easily deteriorates througha switch.

Specifically, a gate electrode of a transistor which easily deterioratesis connected to a wiring to which a high potential is supplied through afirst switching transistor and a wiring to which a low potential issupplied through a second switching transistor; a clock signal is inputto a gate electrode of the first switching transistor; and an invertedclock signal is input to a gate electrode of the second switchingtransistor. Thus, the high potential and the low potential arealternately applied to the gate electrode of the transistor which easilydeteriorates.

Note that various types of switches can be used as a switch shown inthis document (a specification, a claim, a drawing, and the like). Anelectrical switch, a mechanical switch, and the like are given asexamples. That is, any element can be used as long as it can control acurrent flow, without limiting to a certain element. For example, atransistor (e.g., a bipolar transistor or a MOS transistor) a diode(e.g., a PN diode, a PIN diode, a Schottky diode, a MIM (Metal InsulatorMetal) diode, a MIS (Metal Insulator Semiconductor) diode, or adiode-connected transistor), a thyristor, or the like can be used as aswitch. Alternatively, a logic circuit combining such elements can beused as a switch.

In the case of using a transistor as a switch, polarity (a conductivitytype) of the transistor is not particularly limited because it operatesjust as a switch. However, a transistor of polarity with smalleroff-current is preferably used when off-current is to be suppressed. Atransistor provided with an LDD region, a transistor with a multi-gatestructure, and the like are given as examples of a transistor withsmaller off-current. In addition, it is preferable that an N-channeltransistor be used when a potential of a source terminal of thetransistor which is operated as a switch is closer to alow-potential-side power supply (e.g., Vss, GND, or 0 V), while aP-channel transistor be used when the potential of the source terminalis closer to a high-potential-side power supply (e.g., Vdd). This isbecause the absolute value of gate-source voltage can be increased whenthe potential of the source terminal of the transistor which is operatedas the switch is closer to a low-potential-side power supply in anN-channel transistor and when the potential of the source terminal ofthe transistor which is operated as the switch is closer to ahigh-potential-side power supply in a P-channel transistor, so that thetransistor can more accurately operate as a switch. This is also becausea source follower operation is not often performed, so that reduction inoutput voltage does not often occur.

Note that a CMOS switch may be employed by using both N-channel andP-channel transistors. By employing a CMOS switch, the switch can moreprecisely operate as a switch because current can flow when theP-channel transistor or the N-channel transistor is turned on. Forexample, voltage can be appropriately output regardless of whethervoltage of an input signal of the switch is high or low. In addition,since a voltage amplitude value of a signal for turning on or off theswitch can be made small, power consumption can be reduced.

Note also that when a transistor is employed as a switch, the switchincludes an input terminal (one of a source terminal and a drainterminal), an output terminal (the other of the source terminal and thedrain terminal), and a terminal for controlling electrical conduction (agate electrode). On the other hand, when a diode is employed as aswitch, the switch does not have a terminal for controlling electricalconduction in some cases. Therefore, when a diode is used as a switch,the number of wirings for controlling terminals can be more reduced thanthe case of using a transistor as a switch.

Note that in this specification, when it is explicitly described that “Aand B are connected”, the case where elements are electricallyconnected, the case where elements are functionally connected, and thecase where elements are directly connected are included therein. Here,each of A and B corresponds to an object (e.g., a device, an element, acircuit, a wiring, an electrode, a terminal, a conductive film, or alayer). Accordingly, in structures disclosed in this specification,another element may be interposed between elements having a connectionrelation shown in drawings and texts, without limiting to apredetermined connection relation, for example, the connection relationshown in the drawings and the texts.

For example, in the case where A and B are electrically connected, oneor more elements which enable electrical connection of A and B (e.g., aswitch, a transistor, a capacitor, an inductor, a resistor, and/or adiode) may be provided between A and B. In addition, in the case where Aand B are functionally connected, one or more circuits which enablefunctional connection of A and B (e.g., a logic circuit such as aninverter, a NAND circuit, or a NOR circuit, a signal converter circuitsuch as a DA converter circuit, an AD converter circuit, or a gammacorrection circuit, a potential level converter circuit such as a powersupply circuit (e.g., a boosting circuit or a voltage lower controlcircuit) or a level shifter circuit for changing a potential level of asignal, a voltage source, a current source, a switching circuit, or anamplifier circuit such as a circuit which can increase signal amplitude,the amount of current, or the like (e.g., an operational amplifier, adifferential amplifier circuit, a source follower circuit, or a buffercircuit), a signal generating circuit, a memory circuit, and/or acontrol circuit) may be provided between A and B. Alternatively, in thecase where A and B are directly connected, A and B may be directlyconnected without interposing another element or another circuittherebetween.

Note that when it is explicitly described that “A and B are directlyconnected”, the case where A and B are directly connected (i.e., thecase where A and B are connected without interposing another element oranother circuit therebetween) and the case where A and B areelectrically connected (i.e., the case where A and B are connected byinterposing another element or another circuit therebetween) areincluded therein.

Note that when it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected by interposing another element oranother circuit therebetween), the case where A and B are functionallyconnected (i.e., the case where A and B are functionally connected byinterposing another circuit therebetween), and the case where A and Bare directly connected (i.e., the case where A and B are connectedwithout interposing another element or another circuit therebetween) areincluded therein. That is, when it is explicitly described that “A and Bare electrically connected”, the description is the same as the casewhere it is explicitly only described that “A and B are connected”.

Note that a display element, a display device which is a device having adisplay element, a light-emitting element, and a light-emitting devicewhich is a device having a light-emitting element can employ varioustypes and can include various elements. For example, as a displayelement, a display device, a light-emitting element, and alight-emitting device, whose a display medium, contrast, luminance,reflectivity, transmittivity, or the like changes by an electromagneticaction, such as an EL element (e.g., an organic EL element, an inorganicEL element, or an EL element including both organic and inorganicmaterials), an electron emitter, a liquid crystal element, electronicink, an electrophoresis element, a grating light valve (GLV), a plasmadisplay panel (PDP), a digital micromirror device (DMD), a piezoelectricceramic display, or a carbon nanotube can be employed. Note that displaydevices using an EL element include an EL display; display devices usingan electron emitter include a field emission display (FED), an SED-typeflat panel display (SED: Surface-conduction Electron-emitter Display),and the like; display devices using a liquid crystal element include aliquid crystal display (e.g., a transmissive liquid crystal display, asemi-transmissive liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display); and display devices using electronic ink includeelectronic paper.

Note that in this document (the specification, the claim, the drawing,and the like), various types of transistors can be employed as atransistor without limiting to a certain type. For example, a thin filmtransistor (TFT) including a non-single crystalline semiconductor filmtypified by amorphous silicon, polycrystalline silicon, microcrystalline(also referred to as semi-amorphous) silicon, or the like can beemployed. In the case of using the TFT, there are various advantages.For example, since the TFT can be formed at temperature lower than thatof the case of using single crystalline silicon, manufacturing cost canbe reduced and a manufacturing device can be made larger. Since themanufacturing device can be made larger, the TFT can be formed using alarge substrate. Therefore, since many display devices can be formed atthe same time, the TFT can be formed at low cost. In addition, asubstrate having low heat resistance can be used because of lowmanufacturing temperature. Therefore, the transistor can be formed overa light-transmitting substrate. Further, transmission of light in adisplay element can be controlled by using the transistor formed overthe light-transmitting substrate. Alternatively, part of a film whichforms the transistor can transmit light because film thickness of thetransistor is thin. Accordingly, an aperture ratio can be improved.

Note that by using a catalyst (e.g., nickel) in the case of formingpolycrystalline silicon, crystallinity can be further improved and atransistor having excellent electric characteristics can be formed.Accordingly, a gate driver circuit (e.g., a scan line driver circuit), asource driver circuit (e.g., a signal line driver circuit), and a signalprocessing circuit (e.g., a signal generation circuit, a gammacorrection circuit, or a DA converter circuit) can be formed over thesame substrate.

Note that by using a catalyst (e.g., nickel) in the case of formingmicrocrystalline silicon, crystallinity can be further improved and atransistor having excellent electric characteristics can be formed. Atthis time, crystallinity can be improved by performing heat treatmentwithout using a laser. Accordingly, a gate driver circuit (e.g., a scanline driver circuit) and part of a source driver circuit (e.g., ananalog switch) can be formed over the same substrate. In addition, inthe case of not using a laser for crystallization, crystallinityunevenness of silicon can be suppressed. Therefore, an image having highimage quality can be displayed.

Note also that polycrystalline silicon and microcrystalline silicon canbe formed without using a catalyst (e.g., nickel).

In addition, a transistor can be formed by using a semiconductorsubstrate, an SOI substrate, or the like. In that case, a MOStransistor, a junction transistor, a bipolar transistor, or the like canbe used as a transistor described in this specification. Therefore, atransistor with few variations in characteristics, sizes, shapes, or thelike, with high current supply capacity, and with a small size can beformed. By using such a transistor, power consumption of a circuit canbe reduced or a circuit can be highly integrated.

In addition, a transistor including a compound semiconductor or a oxidesemiconductor such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, ITO (Indium TinOxide), or SnO, and a thin film transistor or the like obtained bythinning such a compound semiconductor or a oxide semiconductor can beused. Therefore, manufacturing temperature can be lowered and forexample, such a transistor can be formed at room temperature.Accordingly, the transistor can be formed directly on a substrate havinglow heat resistance such as a plastic substrate or a film substrate.Note that such a compound semiconductor or an oxide semiconductor can beused for not only a channel portion of the transistor but also otherapplications. For example, such a compound semiconductor or an oxidesemiconductor can be used as a resistor, a pixel electrode, or alight-transmitting electrode. Further, since such an element can beformed at the same time as the transistor, cost can be reduced.

A transistor or the like formed by using an inkjet method or a printingmethod can also be used. Accordingly, such a transistor can be formed atroom temperature, can be formed at a low vacuum, or can be formed usinga large substrate. In addition, since the transistor can be formedwithout using a mask (a reticle), layout of the transistor can be easilychanged. Further, since it is not necessary to use a resist, materialcost is reduced and the number of steps can be reduced. Furthermore,since a film is formed only in a necessary portion, a material is notwasted compared with a manufacturing method in which etching isperformed after the film is formed over the entire surface, so that costcan be reduced.

Further, a transistor or the like including an organic semiconductor ora carbon nanotube can be used. Accordingly, such a transistor can beformed using a substrate which can be bent. Therefore, the transistorcan resist a shock.

Furthermore, various transistors can be used.

Moreover, a transistor can be formed using various types of substrates.The type of a substrate is not limited to a certain type. For example, asingle crystalline substrate, an SOI substrate, a glass substrate, aquartz substrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a wood substrate, a cloth substrate(including a natural fiber (e.g., silk, cotton, or hemp), a syntheticfiber (e.g., nylon, polyurethane, or polyester), a regenerated fiber(e.g., acetate, cupra, rayon, or regenerated polyester), or the like), aleather substrate, a rubber substrate, a stainless steel substrate, asubstrate including a stainless steel foil, or the like can be used as asubstrate. Alternatively, a skin (e.g., epidermis or corium) orhypodermal tissue of an animal such as a human being can be used as asubstrate. In addition, the transistor may be formed using onesubstrate, and then, the transistor may be transferred to anothersubstrate. A single crystalline substrate, an SOI substrate, a glasssubstrate, a quartz substrate, a plastic substrate, a paper substrate, acellophane substrate, a stone substrate, a wood substrate, a clothsubstrate (including a natural fiber (e.g., silk, cotton, or hemp), asynthetic fiber (e.g., nylon, polyurethane, or polyester), a regeneratedfiber (e.g., acetate, cupra, rayon, or regenerated polyester), or thelike), a leather substrate, a rubber substrate, a stainless steelsubstrate, a substrate including a stainless steel foil, or the like canbe used as a substrate to which the transistor is transferred.Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissueof an animal such as a human being can be used as a substrate to whichthe transistor is transferred. By using such a substrate, a transistorwith excellent properties or a transistor with low power consumption canbe formed, a device with high durability or high heat resistance can beformed, or reduction in weight can be achieved.

A structure of a transistor can be various modes without limiting to acertain structure. For example, a multi-gate structure having two ormore gate electrodes may be used. When the multi-gate structure is used,a structure where a plurality of transistors are connected in series isprovided because a structure where channel regions are connected inseries is provided. By using the multi-gate structure, off-current canbe reduced or the withstand voltage of the transistor can be increasedto improve reliability. Alternatively, by using the multi-gatestructure, drain-source current does not fluctuate very much even ifdrain-source voltage fluctuates when the transistor operates in asaturation region, so that a flat slope of voltage-currentcharacteristics can be obtained. By utilizing the flat slope of thevoltage-current characteristics, an ideal current source circuit or anactive load having a high resistance value can be realized. Accordingly,a differential circuit or a current mirror circuit having excellentproperties can be realized. In addition, a structure where gateelectrodes are formed above and below a channel may be used. By usingthe structure where gate electrodes are formed above and below thechannel, a channel region is enlarged, so that the amount of currentflowing therethrough can be increased or a depletion layer can be easilyformed to decrease an S value. When the gate electrodes are formed aboveand below the channel, a structure where a plurality of transistors areconnected in parallel is provided.

Further, a structure where a gate electrode is formed above a channel, astructure where a gate electrode is formed below a channel, a staggeredstructure, an inversely staggered structure, a structure where a channelregion is divided into a plurality of regions, or a structure wherechannel regions are connected in parallel or in series can be employed.In addition, a source electrode or a drain electrode may overlap with achannel region (or part of it). By using the structure where the sourceelectrode or the drain electrode may overlap with the channel region (orpart of it), the case can be prevented in which electric charges areaccumulated in part of the channel region, which would result in anunstable operation. Further, an LDD region may be provided. By providingthe LDD region, off-current can be reduced or the withstand voltage ofthe transistor can be increased to improve reliability. Alternatively,drain-source current does not fluctuate very much even if drain-sourcevoltage fluctuates when the transistor operates in the saturationregion, so that a flat slope of voltage-current characteristics can beobtained.

Note that various types of transistors can be used for a transistor inthis specification and the transistor can be formed using various typesof substrates. Accordingly, all of circuits which are necessary torealize a predetermined function may be formed using the same substrate.For example, all of the circuits which are necessary to realize thepredetermined function may be formed using a glass substrate, a plasticsubstrate, a single crystalline substrate, an SOI substrate, or anyother substrate. When all of the circuits which are necessary to realizethe predetermined function are formed using the same substrate, thenumber of component parts can be reduced to cut cost and the number ofconnections to circuit components can be reduced to improve reliability.Alternatively, part of the circuits which are necessary to realize thepredetermined function may be formed using one substrate and anotherpart of the circuits which are necessary to realize the predeterminedfunction may be formed using another substrate. That is, not all of thecircuits which are necessary to realize the predetermined function arerequired to be formed using the same substrate. For example, part of thecircuits which are necessary to realize the predetermined function maybe formed with transistors using a glass substrate and another part ofthe circuits which are necessary to realize the predetermined functionmay be formed using a single crystalline substrate, so that an IC chipformed by a transistor using the single crystalline substrate may beconnected to the glass substrate by COG (Chip On Glass) and the IC chipmay be provided over the glass substrate. Alternatively, the IC chip maybe connected to the glass substrate by TAB (Tape Automated Bonding) or aprinted wiring board. When part of the circuits are formed using thesame substrate in this manner, the number of the component parts can bereduced to cut cost and the number of connections to the circuitcomponents can be reduced to improve reliability. In addition, forexample, by forming a portion with high driving voltage or a portionwith high driving frequency, which consumes large power, using a singlecrystalline substrate and using an IC chip formed by the circuit insteadof forming such a portion using the same substrate, increase in powerconsumption can be prevented.

Note also that one pixel corresponds to one element whose brightness canbe controlled in this specification. Therefore, for example, one pixelcorresponds to one color element and brightness is expressed with theone color element. Accordingly, in the case of a color display devicehaving color elements of R (Red), G (Green), and B (Blue), a minimumunit of an image is formed of three pixels of an R pixel, a G pixel, anda B pixel. Note that the color elements are not limited to three colors,and color elements of more than three colors may be used or a colorother than RGB may be added. For example, RGBW (W corresponds to white)may be used by adding white. In addition, RGB plus one or more colors ofyellow, cyan, magenta emerald green, vermilion, and the like may beused. Further, a color similar to at least one of R, and B may be addedto RGB. For example, R, G, B1, and B2 may be used. Although both B1 andB2 are blue, they have slightly different frequency. Similarly, R1, R2,G, and B may be used. By using such color elements, display which iscloser to the real object can be performed or power consumption can bereduced. Alternatively, as another example, in the case of controllingbrightness of one color element by using a plurality of regions, oneregion may correspond to one pixel. Therefore, for example, in the caseof performing area ratio gray scale display or the case of including asubpixel, a plurality of regions which control brightness are providedin each color element and gray scales are expressed with the wholeregions. In this case, one region which controls brightness maycorrespond to one pixel. Thus, in that case, one color element includesa plurality of pixels. Alternatively, even when the plurality of regionswhich control brightness are provided in one color element, theseregions may be collected as one pixel. Thus, in that case, one colorelement includes one pixel. In that case, one color element includes onepixel. In the case where brightness is controlled in a plurality ofregions in each color element, regions which contribute to display havedifferent area dimensions depending on pixels in some cases. Inaddition, in the plurality of regions which control brightness in eachcolor element, signals supplied to each of the plurality of regions maybe slightly varied to widen a viewing angle. That is, potentials ofpixel electrodes included in the plurality of regions provided in eachcolor element may be different from each other. Accordingly, voltageapplied to liquid crystal molecules are varied depending on the pixelelectrodes. Therefore, the viewing angle can be widened.

Note that when it is explicitly described that “one pixel (for threecolors)”, it corresponds to the case where three pixels of R, G, and Bare considered as one pixel. Meanwhile, when it is explicitly describedthat “one pixel (for one color)”, it corresponds to the case where theplurality of regions are provided in each color element and collectivelyconsidered as one pixel.

Note also that in this document (the specification, the claim, thedrawing, and the like), pixels are provided (arranged) in matrix in somecases. Here, description that pixels are provided (arranged) in matrixincludes the case where the pixels are arranged in a straight line andthe case where the pixels are arranged in a jagged line, in alongitudinal direction or a lateral direction. Therefore, in the case ofperforming full color display with three color elements (e.g., RGB), thefollowing cases are included therein: the case where the pixels arearranged in stripes and the case where dots of the three color elementsare arranged in a delta pattern. In addition, the case is also includedtherein in which dots of the three color elements are provided in Bayerarrangement. Note that the color elements are not limited to threecolors, and color elements of more than three colors may be employed.RGBW (W corresponds to white), RGB plus one or more of yellow, cyan,magenta, and the like, or the like is given as an example. Further, thesizes of display regions may be different between respective dots ofcolor elements. Thus, power consumption can be reduced and the life of adisplay element can be prolonged.

Note also that in this document (the specification, the claim, thedrawing, and the like), an active matrix method in which an activeelement is included in a pixel or a passive matrix method in which anactive element is not included in a pixel can be used.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also various active elements(non-linear elements) can be used. For example, a MIM (Metal InsulatorMetal), a TFD (Thin Film Diode), or the like can also be used. Sincesuch an element has few number of manufacturing steps, manufacturingcost can be reduced or yield can be improved. Further, since size of theelement is small, an aperture ratio can be improved, so that powerconsumption can be reduced or high luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, manufacturing steps is few, so that manufacturing cost can bereduced or the yield can be improved. Further, since an active element(a non-linear element) is not used, the aperture ratio can be improved,so that power consumption can be reduced or high luminance can beachieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor has a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor may change depending on thestructure, the operating condition, and the like of the transistor, itis difficult to define which is a source or a drain. Therefore, in thisspecification, a region functioning as a source and a drain may not becalled the source or the drain. In such a case, for example, one of thesource and the drain may be described as a first terminal and the otherthereof may be described as a second terminal. Alternatively, one of thesource and the drain may be described as a first electrode and the otherthereof may be described as a second electrode. Further alternatively,one of the source and the drain may be described as a source region andthe other thereof may be called a drain region.

Note also that a transistor may be an element having at least threeterminals of a base, an emitter, and a collector. In this case also, oneof the emitter and the collector may be similarly called a firstterminal and the other terminal may be called a second terminal.

A gate corresponds to all or part of a gate electrode and a gate wiring(also referred to as a gate line, a gate signal line, a scan line, ascan signal line, or the like). A gate electrode corresponds to aconductive film which overlaps with a semiconductor which forms achannel region with a gate insulating film interposed therebetween. Notethat part of the gate electrode overlaps with an LDD (Lightly DopedDrain) region, the source region, or the drain region with the gateinsulating film interposed therebetween in some cases. A gate wiringcorresponds to a wiring for connecting a gate electrode of eachtransistor to each other, a wiring for connecting a gate electrode ofeach pixel to each other, or a wiring for connecting a gate electrode toanother wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) which functions as both a gate electrode and a gate wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a gate electrode or a gate wiring. That is, there is aregion where a gate electrode and a gate wiring cannot be clearlydistinguished from each other. For example, in the case where a channelregion overlaps with part of an extended gate wiring, the overlappedportion (region, conductive film, wiring, or the like) functions as botha gate wiring and a gate electrode. Accordingly, such a portion (aregion, a conductive film, a wiring, or the like) may be called either agate electrode or a gate wiring.

In addition, a portion (a region, a conductive film, a wiring, or thelike) which is formed of the same material as a gate electrode, formsthe same island as the gate electrode, and is connected to the gateelectrode may also be called a gate electrode. Similarly, a portion (aregion, a conductive film, a wiring, or the like) which is formed of thesame material as a gate wiring, forms the same island as the gatewiring, and is connected to the gate wiring may also be called a gatewiring. In a strict sense, such a portion (a region, a conductive film,a wiring, or the like) does not overlap with a channel region or doesnot have a function of connecting the gate electrode to another gateelectrode in some cases. However, there is a portion (a region, aconductive film, a wiring, or the like) which is formed of the samematerial as a gate electrode or a gate wiring, forms the same island asthe gate electrode or the gate wiring, and is connected to the gateelectrode or the gate wiring because of conditions in a manufacturingstep. Thus, such a portion (a region, a conductive film, a wiring, orthe like) may also be called either a gate electrode or a gate wiring.

In a multi-gate transistor, for example, a gate electrode is oftenconnected to another gate electrode by using a conductive film which isformed of the same material as the gate electrode. Since such a portion(a region, a conductive film, a wiring, or the like) is a portion (aregion, a conductive film, a wiring, or the like) for connecting thegate electrode to another gate electrode, it may be called a gatewiring, and it may also be called a gate electrode because a multi-gatetransistor can be considered as one transistor. That is, a portion (aregion, a conductive film, a wiring, or the like) which is formed of thesame material as a gate electrode or a gate wiring, forms the sameisland as the gate electrode or the gate wiring, and is connected to thegate electrode or the gate wiring may be called either a gate electrodeor a gate wiring. In addition, for example, part of a conductive filmwhich connects the gate electrode and the gate wiring and is formed of amaterial which is different from that of the gate electrode or the gatewiring may also be called either a gate electrode or a gate wiring.

Note that a gate electrode corresponds to part of a portion (a region, aconductive film, a wiring, or the like) of a gate electrode or a portion(a region, a conductive film, a wiring, or the like) which iselectrically connected to the gate electrode.

Note that when a gate electrode is called a gate wiring, a gate line, agate signal line, a scan line, a scan signal line, there is the case inwhich a gate of a transistor is not connected to a wiring. In this case,the gate wiring, the gate line, the gate signal line, the scan line, orthe scan signal line corresponds to a wiring formed in the same layer asthe gate of the transistor, a wiring formed of the same material of thegate of the transistor, or a wiring formed at the same time as the gateof the transistor in some cases. As examples, a wiring for storagecapacitance, a power supply line, a reference potential supply line, andthe like can be given.

Note also that a source corresponds to all or part of a source region, asource electrode, and a source wiring (also referred to as a sourceline, a source signal line, a data line, a data signal line, or thelike). A source region corresponds to a semiconductor region including alarge amount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Accordingly, a regionincluding a small amount of p-type impurities or n-type impurities,namely, an LDD (Lightly Doped Drain) region is not included in thesource region. A source electrode is part of a conductive layer formedof a material different from that of a source region, and electricallyconnected to the source region. However, there is the case where asource electrode and a source region are collectively called a sourceelectrode. A source wiring is a wiring for connecting a source electrodeof each transistor to each other, a wiring for connecting a sourceelectrode of each pixel to each other, or a wiring for connecting asource electrode to another wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) functioning as both a source electrode and a source wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring. That is, thereis a region where a source electrode and a source wiring cannot beclearly distinguished from each other. For example, in the case where asource region overlaps with part of an extended source wiring, theoverlapped portion (region, conductive film, wiring, or the like)functions as both a source wiring and a source electrode. Accordingly,such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring.

In addition, a portion (a region, a conductive film, a wiring, or thelike) which is formed of the same material as a source electrode, formsthe same island as the source electrode, and is connected to the sourceelectrode, or a portion (a region, a conductive film, a wiring, or thelike) which connects a source electrode and another source electrode mayalso be called a source electrode. Further, a portion which overlapswith a source region may be called a source electrode. Similarly, aportion (a region, a conductive film, a wiring, or the like) which isformed of the same material as a source wiring, forms the same island asthe source wiring, and is connected to the source wiring may also becalled a source wiring. In a strict sense, such a portion (a region, aconductive film, a wiring, or the like) does not have a function ofconnecting the source electrode to another source electrode in somecases. However, there is a portion (a region, a conductive film, awiring, or the like) which is formed of the same material as a sourceelectrode or a source wiring, forms the same island as the sourceelectrode or the source wiring, and is connected to the source electrodeor the source wiring because of conditions in a manufacturing step.Thus, such a portion (a region, a conductive film, a wiring, or thelike) may also be called either a source electrode or a source wiring.

In addition, for example, part of a conductive film which connects asource electrode and a source wiring and is formed of a material whichis different from that of the source electrode or the source wiring maybe called either a source electrode or a source wiring.

Note that a source terminal corresponds to part of a source region, asource electrode, or a portion (a region, a conductive film, a wiring,or the like) which is electrically connected to the source electrode.

Note that when a source electrode is called a source wiring, a sourceline, a source signal line, a data line, a data signal line, there isthe case in which a source (a drain) of a transistor is not connected toa wiring. In this case, the source wiring, the source line, the sourcesignal line, the data line, or the data signal line corresponds to awiring formed in the same layer as the source (the drain) of thetransistor, a wiring formed of the same material of the source (thedrain) of the transistor, or a wiring formed at the same time as thesource (the drain) of the transistor in some cases. As examples, awiring for storage capacitance, a power supply line, a referencepotential supply line, and the like can be given.

Note also that the same can be said for a drain.

Note also that a semiconductor device corresponds to a device having acircuit including a semiconductor element (e.g., a transistor, a diode,or thyristor). The semiconductor device may also include all devicesthat can function by utilizing semiconductor characteristics.

Note also that a display element corresponds to an optical modulationelement, a liquid crystal element, a light-emitting element, an ELelement (an organic EL element, an inorganic EL element, or an ELelement including both organic and inorganic materials), an electronemitter, an electrophoresis element, a discharging element, alight-reflective element, a light diffraction element, a digital microdevice (DMD), or the like. Note that the present invention is notlimited to this.

In addition, a display device corresponds to a device having a displayelement. Note that the display device may also corresponds to a displaypanel itself where a plurality of pixels including display elements areformed over the same substrate as a peripheral driver circuit fordriving the pixels. In addition, the display device may also include aperipheral driver circuit provided over a substrate by wire bonding orbump bonding, namely, an IC chip connected by chip on glass (COG) or anIC chip connected by TAB or the like. Further, the display device mayalso include a flexible printed circuit (FPC) to which an IC chip, aresistor, a capacitor, an inductor, a transistor, or the like isattached. Note also that the display device includes a printed wiringboard (PWB) which is connected through a flexible printed circuit (FPC)and to which an IC chip, a resistor, a capacitor, an inductor, atransistor, or the like is attached. The display device may also includean optical sheet such as a polarizing plate or a retardation plate. Thedisplay device may also include a lighting device, a housing, an audioinput and output device, a light sensor, or the like. Here, a lightingdevice such as a backlight unit may include a light guide plate, a prismsheet, a diffusion sheet, a reflective sheet, a light source (e.g., anLED or a cold cathode fluorescent lamp), a cooling device (e.g., a watercooling device or an air cooling device), or the like.

Moreover, a lighting device corresponds to a device having a backlightunit, a light guide plate, a prism sheet, a diffusion sheet, areflective sheet, or a light source (e.g., an LED, a cold cathodefluorescent lamp, or a hot cathode fluorescent lamp), a cooling device,or the like.

In addition, a light-emitting device corresponds to a device having alight-emitting element and the like.

Note that a reflective device corresponds to a device having alight-reflective element, a light diffraction element, light-reflectiveelectrode, or the like.

A liquid crystal display device corresponds to a display deviceincluding a liquid crystal element. Liquid crystal display devicesinclude a direct-view liquid crystal display, a projection liquidcrystal display, a transmissive liquid crystal display, a reflectiveliquid crystal display, a semi-transmissive liquid crystal display, andthe like.

Note also that a driving device corresponds to a device having asemiconductor element, an electric circuit, or an electronic circuit.For example, a transistor which controls input of a signal from a sourcesignal line to a pixel (also referred to as a selection transistor, aswitching transistor, or the like), a transistor which supplies voltageor current to a pixel electrode, a transistor which supplies voltage orcurrent to a light-emitting element, and the like are examples of thedriving device. A circuit which supplies a signal to a gate signal line(also referred to as a gate driver, a gate line driver circuit, or thelike), a circuit which supplies a signal to a source signal line (alsoreferred to as a source driver, a source line driver circuit, or thelike) are also examples of the driving device.

Note also that a display device, a semiconductor device, a lightingdevice, a cooling device, a light-emitting device, a reflective device,a driving device, and the like overlap with each other in some cases.For example, a display device includes a semiconductor device and alight-emitting device in some cases. Alternatively, a semiconductordevice includes a display device and a driving device in some cases.

In this document (the specification, the claim, the drawing, and thelike), when it is explicitly described that “B is formed on A” or “B isformed over A”, it does not necessarily mean that B is formed in directcontact with A. The description includes the case where A and B are notin direct contact with each other, i.e., the case where another objectis interposed between A and B. Here, each of A and B corresponds to anobject (e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, or a layer).

Accordingly, for example, when it is explicitly described that a layer Bis formed on (or over) a layer A, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or D. Note that another layer (e.g., a layer C or a layer D) maybe a single layer or a plurality of layers.

Similarly, when it is explicitly described that B is formed above A, itdoes not necessarily mean that B is formed in direct contact with A, andanother object may be interposed therebetween. Accordingly, for example,when it is explicitly described that a layer B is formed above a layerA, it includes both the case where the layer B is formed in directcontact with the layer A, and the case where another layer (e.g., alayer C or a layer D) is formed in direct contact with the layer A andthe layer B is formed in direct contact with the layer C or D. Note thatanother layer (e.g., a layer C or a layer D) may be a single layer or aplurality of layers.

Note that when it is explicitly described that B is formed in directcontact with A, it includes not the case where another object isinterposed between A and B but the case where B is formed in directcontact with A.

Note that the same can be said when it is explicitly described that B isformed below or under A.

By using the structure disclosed in this specification, deterioration incharacteristics of all transistors included in a shift register can besuppressed. Therefore, a malfunction of a semiconductor device such as aliquid crystal display device to which the shift register is applied canbe suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are diagrams each showing a structure of a flip-flopshown in Embodiment Mode 1;

FIG. 2 is a timing chart showing operations of the flip-flop shown inFIGS. 1A to 1C;

FIGS. 3A to 3C are diagrams each showing operations of the flip-flopshown in FIGS. 1A to 1C;

FIGS. 4A and 4B are diagrams each showing operations of the flip-flopshown in FIGS. 1A to 1C;

FIGS. 5A to 5C are diagrams each showing a structure of the flip-flopshown in Embodiment Mode 1;

FIG. 6 is a timing chart showing operations of the flip-flop shown inEmbodiment Mode 1;

FIGS. 7A and 7B are diagrams each showing a structure of the flip-flopshown in Embodiment Mode 1;

FIGS. 8A and 8B are diagrams each showing a structure of the flip-flopshown in Embodiment Mode 1;

FIGS. 9A and 9B are diagrams each showing a structure of the flip-flopshown in Embodiment Mode 1;

FIGS. 10A and 10B are diagrams each showing a structure of the flip-flopshown in Embodiment Mode 1;

FIG. 11 is a diagram showing a structure of a shift register shown inEmbodiment Mode 1;

FIG. 12 is a timing chart showing operations of the shift register shownin FIG. 11;

FIG. 13 is a timing chart showing operations of the shift register shownin FIG. 11;

FIG. 14 is a diagram showing a structure of the shift register shown inEmbodiment Mode 1;

FIGS. 15A to 15D are diagrams each showing a structure of a buffer shownin FIG. 14;

FIGS. 16A to 16C are diagrams each showing a structure of the buffershown in FIG. 14;

FIG. 17 is a diagram showing a structure of a display device shown inEmbodiment Mode 1;

FIG. 18 is a timing chart showing writing operations of the displaydevice shown in FIG. 17;

FIG. 19 is a diagram showing a structure of the display device shown inEmbodiment Mode 1;

FIG. 20 is a diagram showing a structure of the display device shown inEmbodiment Mode 1;

FIG. 21 is a timing chart showing writing operations of the displaydevice shown in FIG. 20;

FIG. 22 is a timing chart showing operations of a flip-flop shown inEmbodiment Mode 2;

FIG. 23 is a timing chart showing operations of the flip-flop shown inEmbodiment Mode 2;

FIG. 24 is a diagram showing a structure of a shift register shown inEmbodiment Mode 2;

FIG. 25 is a timing chart showing operations of the shift register shownin FIG. 24;

FIG. 26 is a timing chart showing operations of the shift register shownin FIG. 24;

FIG. 27 is a diagram showing a structure of a display device shown inEmbodiment Mode 2;

FIG. 28 is a diagram showing a structure of the display device shown inEmbodiment Mode 2;

FIG. 29 is a top plan view of the flip-flop in FIG. 7A;

FIGS. 30A and 30B are diagrams each showing a structure of aconventional flip-flop;

FIG. 31 is a diagram showing a structure of a signal line driver circuitshown in Embodiment Mode 5;

FIG. 32 is a timing chart showing operations of the signal line drivercircuit shown in FIG. 31;

FIG. 33 is a diagram showing a structure of the signal line drivercircuit shown in Embodiment Mode 5;

FIG. 34 is a timing chart showing operations of the signal line drivercircuit shown in FIG. 33;

FIG. 35 is a diagram showing a structure of the signal line drivercircuit shown in Embodiment Mode 5;

FIGS. 36A to 36C are diagrams each showing a structure of a protectiondiode shown in Embodiment Mode 6;

FIGS. 37A and 37B are diagrams each showing a structure of theprotection diode shown in Embodiment Mode 6;

FIGS. 38A to 38C are diagrams each showing a structure of the protectiondiode shown in Embodiment Mode 6;

FIGS. 39A to 39C are diagrams each showing a structure of a displaydevice shown in Embodiment Mode 7;

FIG. 40 is a diagram showing a structure of a flip-flop shown inEmbodiment Mode 3;

FIG. 41 is a timing chart showing operations of the flip-flop shown inFIG. 40;

FIG. 42 is a diagram showing a structure of a shift register shown inEmbodiment Mode 3;

FIG. 43 is a timing chart showing operations of the shift register shownin FIG. 42;

FIG. 44 is a diagram showing a structure of a flip-flop shown inEmbodiment Mode 4;

FIG. 45 is a timing chart showing operations of the flip-flop shown inFIG. 44;

FIGS. 46A to 46G are cross-sectional views showing a process for forminga semiconductor device in accordance with the present invention;

FIG. 47 is a cross-sectional view showing a structure of a semiconductordevice in accordance with the present invention;

FIG. 48 is a cross-sectional view showing a structure of a semiconductordevice in accordance with the present invention;

FIG. 49 is a cross-sectional view showing a structure of a semiconductordevice in accordance with the present invention;

FIG. 50 is a cross-sectional view showing a structure of a semiconductordevice in accordance with the present invention;

FIGS. 51A to 51C are graphs each showing a method for driving asemiconductor device in accordance with the present invention;

FIGS. 52A to 52C are graphs each showing a method for driving asemiconductor device in accordance with the present invention;

FIGS. 53A to 53C are diagrams each showing a structure of a displaydevice of a semiconductor device in accordance with the presentinvention;

FIGS. 54A and 54B are diagrams each showing a structure of a peripheralcircuit of a semiconductor device in accordance with the presentinvention;

FIG. 55 is a cross-sectional view showing peripheral components of asemiconductor device in accordance with the present invention;

FIGS. 56A to 56D are views each showing peripheral components of asemiconductor device in accordance with the present invention;

FIG. 57 is a cross-sectional view showing peripheral components of asemiconductor device in accordance with the present invention;

FIGS. 58A to 58C are diagrams each showing a structure of a peripheralcircuit of a semiconductor device in accordance with the presentinvention;

FIG. 59 is a cross-sectional view showing peripheral components of asemiconductor device in accordance with the present invention;

FIGS. 60A and 60B are diagrams each showing a structure of a panelcircuit of a semiconductor device in accordance with the presentinvention;

FIG. 61 is a diagram showing a structure of a panel circuit of asemiconductor device in accordance with the present invention;

FIG. 62 is a diagram showing a structure of a panel circuit of asemiconductor device in accordance with the present invention;

FIGS. 63A and 63B are cross-sectional views of display elements of asemiconductor device in accordance with the present invention;

FIGS. 64A to 64D are cross-sectional views of display elements of asemiconductor device in accordance with the present invention;

FIGS. 65A to 65D are cross-sectional views of display elements of asemiconductor device in accordance with the present invention;

FIGS. 66A to 66D are cross-sectional views of display elements of asemiconductor device in accordance with the present invention;

FIG. 67 is a top plan view of a pixel of a semiconductor device inaccordance with the present invention;

FIGS. 68A and 68B are top plan views of pixels of a semiconductor devicein accordance with the present invention;

FIGS. 69A and 69B are top plan views of pixels of a semiconductor devicein accordance with the present invention;

FIG. 70 is an example of pixel layout of a semiconductor device inaccordance with the present invention;

FIGS. 71A and 71B are examples of pixel layout of a semiconductor devicein accordance with the present invention;

FIGS. 72A and 72B are examples of pixel layout of a semiconductor devicein accordance with the present invention;

FIGS. 73A and 73B are timing charts each showing a method for driving asemiconductor device in accordance with the present invention;

FIGS. 74A and 74B are timing charts each showing a method for driving asemiconductor device in accordance with the present invention;

FIG. 75 is a diagram showing a structure of a pixel of a semiconductordevice in accordance with the present invention;

FIG. 76 is a diagram showing a structure of a pixel of a semiconductordevice in accordance with the present invention;

FIG. 77 is a diagram showing a structure of a pixel of a semiconductordevice in accordance with the present invention;

FIGS. 78A and 78B are an example of pixel layout of a semiconductordevice and a cross-sectional view thereof in accordance with the presentinvention;

FIGS. 79A to 79E are cross-sectional views of display elements of asemiconductor device in accordance with the present invention;

FIGS. 80A to 80C are cross-sectional views of display elements of asemiconductor device in accordance with the present invention;

FIGS. 81A to 81C are cross-sectional views of display elements of asemiconductor device in accordance with the present invention;

FIG. 82 is a view showing a structure of a semiconductor device inaccordance with the present invention;

FIG. 83 is a view showing a structure of a semiconductor device inaccordance with the present invention;

FIG. 84 is a view showing a structure of a semiconductor device inaccordance with the present invention;

FIG. 85 is a view showing a structure of a semiconductor device inaccordance with the present invention;

FIGS. 86A to 86C are views each showing a structure of a semiconductordevice in accordance with the present invention;

FIG. 87 is a view showing a structure of a semiconductor device inaccordance with the present invention;

FIGS. 88A to 88E are diagrams each showing a method for driving asemiconductor device in accordance with the present invention;

FIGS. 89A and 89B are diagrams each showing a method for driving asemiconductor device in accordance with the present invention;

FIGS. 90A to 90C are views and a graph each showing a method for drivinga semiconductor device in accordance with the present invention;

FIGS. 91A and 91B are views each showing a method for driving asemiconductor device in accordance with the present invention;

FIG. 92 is a diagram showing a structure of a semiconductor device inaccordance with the present invention;

FIGS. 93A and 93B are views each showing an electronic device using asemiconductor device in accordance with the present invention;

FIG. 94 is a view showing a structure of a semiconductor device inaccordance with the present invention;

FIGS. 95A to 95C are views each showing an electronic device using asemiconductor device in accordance with the present invention;

FIG. 96 is a view showing an electronic device using a semiconductordevice in accordance with the present invention;

FIG. 97 is a view showing an electronic device using a semiconductordevice in accordance with the present invention;

FIG. 98 is a view showing an electronic device using a semiconductordevice in accordance with the present invention;

FIG. 99 is a view showing an electronic device using a semiconductordevice in accordance with the present invention;

FIGS. 100A and 100B are views each showing an electronic device using asemiconductor device in accordance with the present invention;

FIGS. 101A and 101B are views each showing an electronic device using asemiconductor device in accordance with the present invention;

FIGS. 102A to 102C are views each showing an electronic device using asemiconductor device in accordance with the present invention;

FIGS. 103A and 103B are views each showing an electronic device using asemiconductor device in accordance with the present invention; and

FIG. 104 is a view showing an electronic device using a semiconductordevice in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described by way ofembodiment modes with reference to the drawings. However, the presentinvention can be implemented in various different ways and it will beeasily understood by those skilled in the art that various changes andmodifications are possible. Unless such changes and modifications departfrom the spirit and the scope of the present invention, they should beconstrued as being included therein. Therefore, the present inventionshould not be construed as being limited to the description of theembodiment modes.

Embodiment Mode 1

In this embodiment mode, structures and driving methods of a flip-flop,a driver circuit including the flip-flop, and a display device includingthe driver circuit are described.

A basic structure of a flip-flop of this embodiment mode is describedwith reference to FIG. 1A. A flip-flop shown in FIG. 1A includes a firsttransistor 101, a second transistor 102, a third transistor 103, afourth transistor 104, a fifth transistor 105, a sixth transistor 106, aseventh transistor 107, and an eighth transistor 108. In this embodimentmode, each of the first transistor 101, the second transistor 102, thethird transistor 103, the fourth transistor 104, the fifth transistor105, the sixth transistor 106, the seventh transistor 107, and theeighth transistor 108 is an N-channel transistor and is turned on whengate-source voltage (Vgs) exceeds the threshold voltage (Vth).

Note that in the flip-flop of this embodiment mode, the first transistor101, the second transistor 102, the third transistor 103, the fourthtransistor 104, the fifth transistor 105, the sixth transistor 106, theseventh transistor 107, and the eighth transistor 108 are all N-channeltransistors. Therefore, since amorphous silicon can be used for asemiconductor layer of each transistor in the flip-flop of thisembodiment mode, a manufacturing process can be simplified, and thusmanufacturing cost can be reduced and yield can be improved. Note thateven when polysilicon or single crystalline silicon is used for thesemiconductor layer of each transistor, the manufacturing process can besimplified.

Connection relations of the flip-flop in FIG. 1A are described. A firstelectrode (one of a source electrode and a drain electrode) of the firsttransistor 101 is connected to a fifth wiring 125 and a second electrode(the other of the source electrode and the drain electrode) of the firsttransistor 101 is connected to a third wiring 123. A first electrode ofthe second transistor 102 is connected to a fourth wiring 124; a secondelectrode of the second transistor 102 is connected to the third wiring123; and a gate electrode of the second transistor 102 is connected toan eighth wiring 128. A first electrode of the third transistor 103 isconnected to a sixth wiring 126; a second electrode of the thirdtransistor 103 is connected to a gate electrode of the sixth transistor106; and a gate electrode of the third transistor 103 is connected to aseventh wiring 127. A first electrode of the fourth transistor 104 isconnected to a tenth wiring 130; a second electrode of the fourthtransistor 104 is connected to the gate electrode of the sixthtransistor 106; and a gate electrode of the fourth transistor 104 isconnected to the eighth wiring 128. A first electrode of the fifthtransistor 105 is connected to a ninth wiring 129; a second electrode ofthe fifth transistor 105 is connected to a gate electrode of the firsttransistor 101; and a gate electrode of the fifth transistor 105 isconnected to a first wiring 121. A first electrode of the sixthtransistor 106 is connected to a twelfth wiring 132 and a secondelectrode of the sixth transistor 106 is connected to the gate electrodeof the first transistor 101. A first electrode of the seventh transistor107 is connected to a thirteenth wiring 133; a second electrode of theseventh transistor 107 is connected to the gate electrode of the firsttransistor 101; and a gate electrode of the seventh transistor 107 isconnected to a second wiring 122. A first electrode of the eighthtransistor 108 is connected to an eleventh wiring 131; a secondelectrode of the eighth transistor 108 is connected to the gateelectrode of the sixth transistor 106; and a gate electrode of theeighth transistor 108 is connected to the gate electrode of the firsttransistor 101.

Note that a connection point of the gate electrode of the firsttransistor 101, the second electrode of the sixth transistor 106, thesecond electrode of the seventh transistor 107, and the gate electrodeof the eighth transistor 108 is denoted by a node 141. Further, aconnection point of the second electrode of the third transistor 103,the second electrode of the fourth transistor 104, the gate electrode ofthe sixth transistor 106, and the second electrode of the eighthtransistor 108 is denoted by a node 142.

Note that the first wiring 121, the second wiring 122, the third wiring123, the fifth wiring 125, the seventh wiring 127, and the eighth wiring128 may be referred to as a first signal line, a second signal line, athird signal line, a fourth signal line, a fifth signal line, and asixth signal line, respectively. Further, the fourth wiring 124, thesixth wiring 126, the ninth wiring 129, the tenth wiring 130, theeleventh wiring 131, the twelfth wiring 132, and the thirteenth wiring133 may be referred to as a first power supply line, a second powersupply line, a third power supply line, a fourth power supply line, afifth power supply line, a sixth power supply line, and a seventh powersupply line, respectively.

Next, operations of the flip-flop shown in FIG. 1A are described withreference to a timing chart in FIG. 2, and FIGS. 3A to 4B. Note that thetiming chart in FIG. 2 is described by dividing the whole period into aset period, a selection period, a reset period, a first non-selectionperiod, and a second non-selection period. Note also that the setperiod, the reset period, the first non-selection period, and the secondnon-selection period are collectively referred to as a non-selectionperiod in some cases.

Note that a potential of V1 is supplied to the sixth wiring 126 and theninth wiring 129, and a potential of V2 is supplied to the fourth wiring124, the tenth wiring 130, the eleventh wiring 131, the twelfth wiring132, and the thirteenth wiring 133. Here, V1>V2 is satisfied.

Note that a signal 221, a signal 225, a signal 228, a signal 227, and asignal 222 shown in FIG. 2 are input to the first wiring 121, the fifthwiring 125, the eighth wiring 128, the seventh wiring 127, and thesecond wiring 122, respectively. In addition, a signal 223 shown in FIG.2 is output from the third wiring 123. Here, each of the signal 221, thesignal 225, the signal 228, the signal 227, the signal 222, and thesignal 223 is a digital signal in which a potential of an H-level signalis at V1 (hereinafter also referred to as an H level) and a potential ofan L-level signal is at V2 (hereinafter also referred to as an L level).Further, the signal 221, the signal 225, the signal 228, the signal 227,the signal 222, and the signal 223 may be referred to as a start signal,a power clock signal (PCK), a first control clock signal (CCK1), asecond control clock signal (CCK2), a reset signal, and an outputsignal, respectively.

Note that any signal, potential, or current may be input to each of thefirst wiring 121, the second wiring 122, the fourth wiring 124, thefifth wiring 125, the sixth wiring 126, the seventh wiring 127, theeighth wiring 128, the ninth wiring 129, the tenth wiring 130, theeleventh wiring 131, the twelfth wiring 132, and the thirteenth wiring133.

First, in the set period shown in period A of FIG. 2 and FIG. 3A, thesignal 221 becomes an H level and the fifth transistor 105 is turned on;the seventh transistor 107 is turned off because the signal 222 is at anL level; the signal 228 becomes an H level and the second transistor 102and the fourth transistor 104 are turned on; and the signal 227 becomesan L level and the third transistor 103 is turned off. A potential ofthe node 141 (a potential 241) at this time becomes V1−Vth105 (Vth105corresponds to the threshold voltage of the fifth transistor 105)because the second electrode of the fifth transistor 105 corresponds tothe source electrode and the potential of the node 141 (the potential241) becomes a value obtained by subtracting the threshold voltage ofthe fifth transistor 105 from a potential of the ninth wiring 129. Thus,the first transistor 101 and the eighth transistor 108 are turned on andthe fifth transistor 105 is turned off. A potential of the node 142 (apotential 242) at this time becomes V2 and the sixth transistor 106 isturned off. Since the third wiring 123 is connected to the fifth wiring125 to which an L-level signal is input and the fourth wiring 124 towhich V2 is supplied in the set period in this manner, a potential ofthe third wiring 123 becomes V2. Therefore, an L-level signal is outputfrom the third wiring 123. Further, the node 141 enters into a floatingstate while being kept at V1−Vth105.

Note that the flip-flop of this embodiment mode can perform operationswhich are similar to those in the above-described set period even whenthe first electrode of the fifth transistor 105 is connected to thefirst wiring 121 as shown in FIG. 5A. Since the ninth wiring 129 is notnecessary in a flip-flop in FIG. 5A, yield can be improved. Further, inthe flip-flop in FIG. 5A, a layout area can be reduced.

Note that in the flip-flop of this embodiment mode, a transistor 501 maybe additionally provided as shown in FIG. 5C. A first electrode of thetransistor 501 is connected to a wiring 511 to which V2 is supplied; asecond electrode of the transistor 501 is connected to the node 141; anda gate electrode of the transistor 501 is connected to the first wiring121. Since time at which the potential of the node 142 lowers can beshortened by the transistor 501 in a flip-flop in FIG. 5C, the sixthtransistor 106 can be turned off quickly. Therefore, since time at whichthe potential of the node 141 becomes V1−Vth105 can be shortened in theflip-flop in FIG. 5C, high speed operation can be performed and theflip-flop in FIG. 5C can be applied to a larger display device or ahigher-definition display device.

In the selection period shown in period B of FIG. 2 and FIG. 3B, thesignal 221 becomes an L level and the fifth transistor 105 is turnedoff; the seventh transistor 107 remains off because the signal 222remains at an L level; the signal 228 becomes an L level and the secondtransistor 102 and the fourth transistor 104 are turned off; and thesignal 227 becomes an H level and the third transistor 103 is turned on.The node 141 at this time remains at V1−Vth105. Thus, the firsttransistor 101 and the eighth transistor 108 remain on. The potential ofthe node 142 at this time becomes V2+β (β corresponds to a givenpositive number) because a potential difference (V1−V2) between apotential of the eleventh wiring 131 (V2) and a potential of the sixthwiring 126 (V1) is voltage divided by the third transistor 103 and theeighth transistor 108. Further, β<Vth106 (the threshold voltage of thesixth transistor 106) is satisfied. Thus, the sixth transistor 106remains off. Here, since an H-level signal is input to the fifth wiring125, the potential of the third wiring 123 starts to rise. Then, thepotential of the node 141 rises from V1−Vth105 by a bootstrap operationand becomes V1+Vth101+α (Vth 101 corresponds to the threshold voltage ofthe first transistor and a corresponds to a given positive number).Therefore, the potential of the third wiring 123 becomes V1 because itbecomes a potential which is equal to that of the fifth wiring 125.Since the third wiring 123 is connected to the fifth wiring 125 to whichthe H-level signal is supplied in the selection period in this manner,the potential of the third wiring 123 becomes V1. Therefore, an H-levelsignal is output from the third wiring 123.

Note that this bootstrap operation is performed by capacitive couplingof parasitic capacitance between the gate electrode and the secondelectrode of the first transistor 101. Note also that the bootstrapoperation can be stably performed by providing a capacitor 151 betweenthe gate electrode and the second electrode of the first transistor 101as shown in FIG. 1B, and the parasitic capacitance of the firsttransistor 101 can be reduced. Here, in the capacitor 151, a gateinsulating film may be used as an insulating layer and a gate electrodelayer and a wiring layer may be used as conductive layers; a gateinsulating film may be used as the insulating layer and a gate electrodelayer and a semiconductor layer to which an impurity is added may beused as the conductive layers; or an interlayer film (an insulatingfilm) may be used as the insulating layer and a wiring layer and alight-transmitting electrode layer may be used as the conductive layers.Note also that when a gate electrode layer and a wiring layer are usedas the conductive layers in the capacitor 151, it is preferable that thegate electrode layer be connected to the gate electrode of the firsttransistor 101 and the wiring layer be connected to the second electrodeof the first transistor 101. When a gate electrode layer and a wiringlayer are used as the conductive layers, it is more preferable that thegate electrode layer be directly connected to the gate electrode of thefirst transistor 101 and the wiring layer be directly connected to thesecond electrode of the first transistor 101. This is because increasein a layout area of the flip-flop due to provision of the capacitor 151is suppressed.

Further, as shown in FIG. 1C, a transistor 152 may be used as thecapacitor 151. A gate electrode of the transistor 152 is connected tothe node 141 and a first electrode and a second electrode of thetransistor 152 are connected to the third wiring 123, so that thetransistor 152 can function as a capacitor having a large capacitancecomponent. Note that the transistor 152 can function as a capacitor evenwhen one of the first electrode and the second electrode of thetransistor 152 is in a floating state.

Note that it is necessary that the first transistor 101 supply anH-level signal to the third wiring 123. Therefore, in order to shortenfall time and rise time of the signal 223, it is preferable that thefirst transistor 101 have the largest value of W/L (a ratio of a channelwidth W to a channel length L) among the first transistor 101 to eighthtransistor 108.

Further, since it is necessary that the fifth transistor 105 set thepotential of the node 141 (the gate electrode of the first transistor101) at V1−Vth105 in the set period, a value of W/L of the fifthtransistor 105 is preferably, ½ to ⅕ times, more preferably, ⅓ to ¼times the value of W/L of the first transistor 101.

In order to set the potential of the node 142 at V2−β, it is preferablethat a value of W/L (a ratio of a channel width W to a channel length L)of the eighth transistor 108 be at least ten times a value of W/L of thethird transistor 103. Therefore, a transistor size (W×L) of the eighthtransistor 108 is increased. Here, by setting the value of the channellength L of the third transistor 103 longer than the channel length L ofthe eighth transistor 108, preferably, two to three times the channellength L of the eighth transistor 108, the transistor size of the eighthtransistor 108 can be decreased. Therefore, a layout area can bereduced.

In the reset period shown in period C of FIG. 2 and FIG. 3C, the fifthtransistor 105 remains off because the signal 221 remains at an L level;the signal 222 becomes an H level and the seventh transistor 107 isturned on; the signal 228 becomes an H level and the second transistor102 and the fourth transistor 104 are turned on; and the signal 227becomes an L level and the third transistor 103 is turned off. Thepotential of the node 141 at this time becomes V2 because a potential ofthe thirteenth wiring 133 is supplied through the seventh transistor107. Thus, the first transistor 101 and the eighth transistor 108 areturned off. The potential of the node 142 at this time becomes V2because the fourth transistor 104 is turned on. Thus, the sixthtransistor 106 is turned off. Since the third wiring 123 is connected tothe fourth wiring 124 to which V2 is supplied in the reset period inthis manner, the potential of the third wiring 123 becomes V2.Therefore, an L-level signal is output from the third wiring 123.

Note that by delaying timing at which the seventh transistor 107 isturned on, the fall time of the signal 223 can be shortened. This isbecause an L-level signal which is input to the fifth wiring 125 can besupplied to the third wiring 123 through the first transistor 101 havinga larger value of W/L.

Alternatively, by decreasing the value of W/L of the seventh transistor107 and lengthening fall time which is necessary for the potential ofthe node 141 to become V2, the fall time of the signal 223 can also beshortened. In this case, the value of W/L of the seventh transistor 107is preferably, 1/10 to 1/40 times, more preferably, 1/20 to 1/30 timesthe value W/L of the first transistor 101.

Note that operations which are similar to those in the above-describedreset period can be performed even when the seventh transistor 107 isnot provided as shown in FIG. 5B. Since the transistor and the wiringscan be reduced in the flip-flop in FIG. 5B, a layout area can bereduced.

In the first non-selection period shown in period D of FIG. 2 and FIG.4A, the fifth transistor 105 remains off because the signal 221 remainsat an L level; the signal 222 becomes an L level and the seventhtransistor 107 is turned off; the signal 228 becomes an L level and thesecond transistor 102 and the fourth transistor 104 are turned off; andthe signal 227 becomes an H level and the third transistor 103 is turnedon. The potential of the node 142 at this time becomes V1−Vth103 (Vth103corresponds to the threshold voltage of the third transistor 103)because the second electrode of the third transistor 103 corresponds tothe source electrode and the potential of the node 142 becomes a valueobtained by subtracting the threshold voltage of the third transistor103 from a potential of the seventh wiring 127 (V1). Thus, the sixthtransistor 106 is turned on. The potential of the node 141 at this timebecomes V2 because the sixth transistor 106 is turned on. Thus, thefirst transistor 101 and the eighth transistor 108 remain off. In thismanner, in the first non-selection period, the third wiring 123 entersinto a floating state and remains at V2.

Note that each of the flip-flops of this embodiment mode can suppress athreshold voltage shift of the second transistor 102 by truing off thesecond transistor 102.

Note that a threshold voltage shift of the third transistor 103 can besuppressed by setting a potential of the signal 227 at V1 or less andlowering a potential of the gate electrode of the third transistor 103.Further, a threshold voltage shift of the fourth transistor 104 and thethreshold voltage shift of the second transistor 102 can be suppressedby setting a potential of the signal 228 at V2 or less and applyingreverse bias voltage to the fourth transistor 104 and the secondtransistor 102.

Note also that V2 can be supplied to the third wiring 123 byadditionally providing a transistor 901 as shown in FIG. 9A. A firstelectrode of the transistor 901 is connected to the fourth wiring 124; asecond electrode of the transistor 901 is connected to the third wiring123; and a gate electrode of the transistor 901 is connected to the node142. Therefore, on/off of the transistor 901 is controlled at the sametiming as the sixth transistor 106. Accordingly, since the third wiring123 does not enter into a floating state, a flip-flop in FIG. 9A canresist noise. Further, the transistor 901 can be provided instead of thesecond transistor 102 as shown in FIG. 9B.

In the second non-selection period shown in period E of FIG. 2 and FIG.4B, the fifth transistor 105 remains off because the signal 221 remainsat an L level; the seventh transistor 107 remains off because the signal222 remains at an L level; the signal 228 becomes an H level and thesecond transistor 102 and the fourth transistor 104 are turned on; andthe signal 227 becomes an L level and the third transistor 103 is turnedoff. The potential of the node 142 at this time becomes V2 because thefourth transistor 104 is turned on. Thus, the sixth transistor 106 isturned off. The node 141 at this time remains at V2 because the node 141enters into a floating state. Thus, the first transistor 101 and theeighth transistor 108 remain off. Since the third wiring 123 isconnected to the fourth wiring 124 to which V2 is supplied in the secondnon-selection period in this manner, the potential of the third wiring123 becomes V2. Therefore, an L-level signal is output from the thirdwiring 123.

Note that each of the flip-flops of this embodiment mode can suppress athreshold voltage shift of the sixth transistor 106 by truing off thesixth transistor 106.

Note that in each of the flip-flops of this embodiment mode, thepotential of the third wiring 123 can be set at V2 in the secondnon-selection period even when the potential of the third wiring 123fluctuates due to noise. Further, in each of the flip-flops of thisembodiment mode, the potential of the node 141 can be set at V2 in thefirst non-selection period even when the potential of the node 141fluctuates due to noise.

Note that the threshold voltage shift of the third transistor 103 can besuppressed by setting the potential of the signal 227 at V2 or less andapplying reverse bias voltage to the third transistor 103. Further, thethreshold voltage shift of the fourth transistor 104 and the thresholdvoltage shift of the second transistor 102 can be suppressed by settingthe potential of the signal 228 at V1 or less and lowering a potentialof the gate electrode of the fourth transistor 104 and a potential ofthe gate electrode of the second transistor 102.

As described above, since the threshold voltage shift of the secondtransistor 102 and the threshold voltage shift of the sixth transistor106 can be suppressed in each of the flip-flops of this embodiment mode,the life can be prolonged. In addition, since threshold voltage shiftsof all the transistors can be suppressed in each of the flip-flops ofthis embodiment mode, the life can be prolonged. Further, since each ofthe flip-flops of this embodiment mode can resist noise, reliability canbe improved.

Here, functions of the first transistor 101 to the eighth transistor 108are described. The first transistor 101 has a function of selectingtiming for supplying the potential of the fifth wiring 125 to the thirdwiring 123 and raising the potential of the node 141 by the bootstrapoperation and functions as a bootstrap transistor. The second transistor102 has a function of selecting timing for supplying the potential ofthe fourth wiring 124 to the third wiring 123 and functions as aswitching transistor. The third transistor 103 has a function ofselecting timing for supplying the potential of the sixth wiring 126 tothe node 142 and functions as a switching transistor. The fourthtransistor 104 has a function of selecting timing for supplying apotential of the tenth wiring 130 to the node 142 and functions as aswitching transistor. The fifth transistor 105 has a function ofselecting timing for supplying the potential of the ninth wiring 129 tothe node 141 and functions as a transistor for input. The sixthtransistor 106 has a function of selecting timing for supplying apotential of the twelfth wiring 132 to the node 141 and functions as aswitching transistor. The seventh transistor 107 has a function ofselecting timing for supplying the potential of the thirteenth wiring133 to the node 141 and functions as a switching transistor. The eighthtransistor 108 has a function of selecting timing for supplying thepotential of the eleventh wiring 131 to the node 142 and functions as aswitching transistor.

Note that the first transistor 101 to the eighth transistor 108 are notlimited to transistors as long as they have the above-describedfunctions. For example, a diode, a CMOS analog switch, any logiccircuit, or the like may be applied to each of the second transistor102, the third transistor 103, the fourth transistor 104, the sixthtransistor 106, the seventh transistor 107, and the eighth transistor108 functioning as the switching transistor as long as it is an elementhaving a switching function. Further, a PN junction diode, adiode-connected transistor, or the like may be applied to the fifthtransistor 105 functioning as the transistor for input as long as it hasa function of selecting timing at which the potential of the node 141 israised to be turned off.

Note that arrangement, the number, and the like of the transistors arenot limited to those of FIG. 1A as long as operations which are similarto those of FIG. 1A are performed. As is apparent from FIGS. 3A to 4Bwhich show the operations of the flip-flop in FIG. 1A, in thisembodiment mode, it is only necessary to have electrical continuity inthe set period, the selection period, the reset period, the firstnon-selection period, and the second non-selection period, as shown by asolid line in each of FIGS. 3A to 4B. Thus, a transistor, anotherelement (e.g., a resistor or a capacitor), a diode, a switch, any logiccircuit, or the like may be additionally provided as long as a structureis employed in which a transistor or the like is provided so as tosatisfy the above-described conditions and the structure can beoperated.

For example, the potential of the node 142 is determined whether to turnon the third transistor 103 or turn on the fourth the fourth transistor104. However, by connecting a resistor 1011 and a resistor 1012 betweenthe seventh wiring 127 and the eighth wiring 128 as shown in FIG. 10A,operations which are similar to those of FIG. 1A can also be performed.Since the number of the transistors and the number of the wirings can bereduced in a flip-flop in FIG. 10A, reduction in a layout area,improvement in yield, and the like can be achieved.

Further, as shown in FIG. 10B, instead of providing the resistor 1011, adiode-connected transistor 1021 and a diode-connected transistor 1022may be provided between the seventh wiring 127 and the node 142, andinstead of providing the resistor 1012, a diode-connected transistor1023 and a diode-connected transistor 1024 may be provided between theeighth wiring 128 and the node 142. A first electrode of the transistor1021, a gate electrode of the transistor 1021, and a first electrode ofthe transistor 1022 are connected to the seventh wiring 127. A firstelectrode of the transistor 1023, a first electrode of the transistor1024, and a gate electrode of the transistor 1024 are connected to theeighth wiring 128. A second electrode of the transistor 1021, a secondelectrode of the transistor 1022, a gate electrode of the transistor1022, a second electrode of the transistor 1023, a gate electrode of thetransistor 1023, and a second electrode of the transistor 1024 areconnected to the node 142. That is, two diodes are connected reverselyand in parallel between the seventh wiring 127 and the node 142, and twodiodes are connected reversely and in parallel between the eighth wiring128 and the node 142.

Note that drive timing of the flip-flop of this embodiment mode is notlimited to that of FIG. 2 as long as operations which are similar tothose of FIGS. 1A to 1C are performed.

For example, as shown in a timing chart shown in FIG. 6, a period forinputting an H-level signal to each of the first wiring 121, the secondwiring 122, the fifth wiring 125, the seventh wiring 127, and the eighthwiring 128 may be shortened. In FIG. 6, timing at which a signal isswitched from an L level to an H level is delayed for a period Ta1 andtiming at which a signal is switched from an H level to an L levelbecomes early by a period Ta2, compared with the timing chart in FIG. 2.Therefore, instantaneous current of each wiring is made small in aflip-flop to which the timing chart in FIG. 6 is applied, so that powersaving, suppression of a malfunction, improvement in a range ofoperating conditions, or/and the like can be achieved. Further, in theflip-flop which employs the timing chart in FIG. 6, fall time of asignal which is output from the third wiring 123 can be shortened in areset period. This is because timing at which the potential of the node141 becomes an L level is delayed for the period Ta1+the period Ta2, andthus an L-level signal which is input to the fifth wiring 125 issupplied to the third wiring 123 through the first transistor 101 havinghigh current supply capacity (having a wide channel width). Note thatportions which are common to those of the timing chart in FIG. 2 aredenoted by common reference numerals and description thereof is omitted.

Note that a relation among the period Ta1, the period Ta2, and a periodTb preferably satisfies ((Ta1+Ta2)/(Ta1+Ta2+Tb))×100<10[%]. Morepreferably, the relation among the period Ta1, the period Ta2, and theperiod Tb satisfies ((Ta1+Ta2)(Ta1+Ta2+Tb))×100<5[%]. In addition, it ispreferable to set the period Ta1 the period Ta2.

Note that the first wiring 121 to the thirteenth wiring 133 can befreely connected as long as operations which are similar to those ofFIGS. 1A to 1C are performed. For example, as shown in FIG. 7A, thefirst electrode of the second transistor 102, the first electrode of thefourth transistor 104, the first electrode of the sixth transistor 106,the first electrode of the seventh transistor 107, and the firstelectrode of the eighth transistor 108 may be connected to a seventhwiring 707. In addition, the first electrode of the fifth transistor 105and the first electrode of the third transistor 103 may be connected toa sixth wiring 706. Further, the gate electrode of the second transistor102 and the gate electrode of the fourth transistor 104 may be connectedto a fifth wiring 705. Furthermore, the first electrode of the firsttransistor 101 and the gate electrode of the third transistor 103 may beconnected to a fourth wiring 704. Note that as shown in FIG. 7B, thefirst electrode of the first transistor 101 may be connected to aneighth wiring 708. In addition, as shown in FIG. 8A, the first electrodeof the third transistor 103 may be connected to a ninth wiring 709.Further, as shown in FIG. 8B, the first electrode of the fourthtransistor 104 may be connected to a tenth wiring 710. Note also thatportions which are common to those of FIGS. 1A to 1C are denoted bycommon reference numerals and description thereof is omitted.

Since the number of the wirings can be reduced in a flip-flop in FIG.7A, yield can be improved, a layout area can be reduced, reliability canbe improved, or a range of operating conditions can be improved. Inaddition, since a potential which is applied to the third transistor 103is lowered and a reverse bias voltage can be applied in a flip-flop inFIG. 7B, the threshold voltage shift of the third transistor 103 can befurther suppressed. Further, since a potential which is supplied to theninth wiring 709 can be lowered in a flip-flop in FIG. 8A, the thresholdvoltage shift of the sixth transistor 106 can be further suppressed.Furthermore, since current flowing through the third transistor 103 andthe fourth transistor 104 can be set so as not to adversely affect theoperations of other transistors, a range of operating conditions can beimproved.

FIG. 29 shows an example of a top plan view of the flip-flop shown inFIG. 7A. A conductive layer 2901 has a portion functioning as the firstelectrode of the first transistor 101 and is connected to the fourthwiring 704 through a wiring 2951. A conductive layer 2902 has a functionas the second electrode of the first transistor 101 and is connected toa third wiring 703 through a wiring 2952. A conductive layer 2903 hasfunctions as the gate electrode of the first transistor 101 and the gateelectrode of the eighth transistor 108. A conductive layer 2904 has aportion functioning as the second electrode of the second transistor 102and is connected to the third wiring 703 through the wiring 2952. Aconductive layer 2905 has functions as the first electrode of the secondtransistor 102, the first electrode of the fourth transistor 104, thefirst electrode of the sixth transistor 106, and the first electrode ofthe eighth transistor 108 and is connected to the seventh wiring 707. Aconductive layer 2906 has functions as the gate electrode of the secondtransistor 102 and the gate electrode of the fourth transistor 104 andis connected to the fifth wiring 705 through a wiring 2953. A conductivelayer 2907 has a function as the first electrode of the third transistor103 and is connected to the sixth wiring 706 through a wiring 2954. Aconductive layer 2908 has functions as the second electrode of the thirdtransistor 103, the second electrode of the fourth transistor 104, andthe second electrode of the eighth transistor 108. A conductive layer2909 has a function as the gate electrode of the third transistor 103and is connected to the fourth wiring 704 through a wiring 2955. Aconductive layer has a function as the first electrode of the fifthtransistor 105 and is connected to the sixth wiring 706 through a wiring2956. A conductive layer 2911 has functions as the second electrode ofthe fifth transistor 105 and the second electrode of the seventhtransistor 107 and is connected to the conductive layer 2903 through awiring 2957. A conductive layer 2912 has a function as the gateelectrode of the fifth transistor 105 and is connected to a first wiring701 through a wiring 2958. A conductive layer 2913 has a function as thesecond electrode of the sixth transistor 106 and is connected to theconductive layer 2903 through a wiring 2959. A conductive layer 2914 hasa function as the gate electrode of the sixth transistor 106 and isconnected to the conductive layer 2908 through a wiring 2961. Aconductive layer 2915 has a function as the second electrode of theseventh transistor 107 and is connected to the seventh wiring 707. Aconductive layer 2916 has a function as the gate electrode of theseventh transistor 107 and is connected to a second wiring 702 through awiring 2960.

Here, the wiring 2960 has a smaller wiring width than that of the wiring2951, the wiring 2952, the wiring 2953, the wiring 2954, the wiring2955, the wiring 2956, the wiring 2957, the wiring 2958, the wiring2959, or the wiring 2961. Alternatively, the wiring 2960 has longerwiring length than that of the wiring 2951, the wiring 2952, the wiring2953, the wiring 2954, the wiring 2955, the wiring 2956, the wiring2957, the wiring 2958, the wiring 2959, or the wiring 2961. That is, aresistance value of the wiring 2960 is increased. Thus, timing at whicha potential of the conductive layer 2916 becomes an H level can bedelayed in the reset period. Therefore, since timing at which theseventh transistor 107 is turned on can be delayed in the reset period,a signal of the third wiring 703 can be quickly set at an L level. Thisis because timing at which the node 141 becomes an L level is delayedand an L-level signal is supplied to the third wiring 703 through thefirst transistor 101 in that delay period.

Note that the wiring 2951, the wiring 2952, the wiring 2953, the wiring2954, the wiring 2955, the wiring 2956, the wiring 2957, the wiring2958, the wiring 2959, the wiring 2960, and the wiring 2961 are similarto a pixel electrode (or referred to as a light-transmitting electrodeor a reflective electrode) and are formed by using a similar material ina similar process.

Note that portions functioning as the gate electrode, the firstelectrode, and the second electrode of the first transistor 101correspond to portions where the conductive layers having the gateelectrode, the first electrode, and the second electrode of the firsttransistor 101 overlap with a semiconductor layer 2981. Portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the first transistor 102 correspond to portions where theconductive layers having the gate electrode, the first electrode, andthe second electrode of the first transistor 102 overlap with asemiconductor layer 2982. Portions functioning as the gate electrode,the first electrode, and the second electrode of the first transistor103 correspond to portions where the conductive layers having the gateelectrode, the first electrode, and the second electrode of the firsttransistor 103 overlap with a semiconductor layer 2983. Portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the first transistor 104 correspond to portions where theconductive layers having the gate electrode, the first electrode, andthe second electrode of the first transistor 104 overlap with asemiconductor layer 2984. Portions functioning as the gate electrode,the first electrode, and the second electrode of the first transistor105 correspond to portions where the conductive layers having the gateelectrode, the first electrode, and the second electrode of the firsttransistor 105 overlap with a semiconductor layer 2985. Portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the first transistor 106 correspond to portions where theconductive layers having the gate electrode, the first electrode, andthe second electrode of the first transistor 106 overlap with asemiconductor layer 2986. Portions functioning as the gate electrode,the first electrode, and the second electrode of the first transistor107 correspond to portions where the conductive layers having the gateelectrode, the first electrode, and the second electrode of the firsttransistor 107 overlap with a semiconductor layer 2987. Portionsfunctioning as the gate electrode, the first electrode, and the secondelectrode of the first transistor 108 correspond to portions where theconductive layers having the gate electrode, the first electrode, andthe second electrode of the first transistor 108 overlap with asemiconductor layer 2988.

Next, a structure and a driving method of a shift register including theabove-described flip-flop of this embodiment mode are described.

The structure of the shift register of this embodiment mode is describedwith reference to FIG. 11. The shift register in FIG. 11 includes npieces of flip-flops (flip-flops 1101_1 to 1101_n).

Connection relations of the shift register in FIG. 11 are described. Ina flip-flop 1101_i of an i-th stage (any one of the flip-flops 1101_1 to1101_n) of the shift register in FIG. 11, the first wiring 121 shown inFIG. 1A is connected to a seventh wiring 1117_i−1; the second wiring 122shown in FIG. 1A is connected to a seventh wiring 1117_i+1; the thirdwiring 123 shown in FIG. 1A is connected to a seventh wiring 1117_i; thefourth wiring 124, the tenth wiring 130, the eleventh wiring 131, thetwelfth wiring 132, and the thirteenth wiring 133 shown in FIG. 1A areconnected to a fifth wiring 1115; the fifth wiring 125 and the seventhwiring 127 shown in FIG. 1A are connected to a second wiring 1112 in aflip-flop of an odd-numbered stage; the fifth wiring 125 and the seventhwiring 127 shown in FIG. 1A are connected to a third wiring 1113 in aflip-flop of an even-numbered stage; the eighth wiring 128 shown in FIG.1A is connected to the third wiring 1113 in a flip-flop of anodd-numbered stage; the eighth wiring 128 shown in FIG. 1A is connectedto the second wiring 1112 in a flip-flop of an even-numbered stage; andthe sixth wiring 126 and the ninth wiring 129 shown in FIG. 1A areconnected to a fourth wiring 1114. Note that the first wiring 121 shownin FIG. 1A of the flip-flop 1101_1 of a first stage is connected to afirst wiring 1111, and the second wiring 122 shown in FIG. 1A of theflip-flop 1101_n of an n-th stage is connected to a sixth wiring 1116.

Note that the first wiring 1111, the second wiring 1112, the thirdwiring 1113, and the sixth wiring 1116 may be referred to as a firstsignal line, a second signal line, a third signal line, and a fourthsignal line, respectively. Further, the fourth wiring 1114 and the fifthwiring 1115 may be referred to as a first power supply line and a secondpower supply line, respectively.

Next, operations of the shift register shown in FIG. 11 are describedwith reference to a timing chart in FIG. 12 and a timing chart in FIG.13. Here, the timing chart in FIG. 12 is divided into a scanninginterval and a retrace interval. The scanning interval corresponds to aninterval from time when output of a selection signal from the seventhwiring 1117_1 is started to time when output of a selection signal froma seventh wiring 1117_n is completed. The retrace interval correspondsto an interval from time when output of the selection signal from theseventh wiring 1117_n is completed to time when output of the selectionsignal from the seventh wiring 1117_1 is started.

Note that the potential of V1 is supplied to the fourth wiring 1114 andthe potential of V2 is supplied to the fifth wiring 1115.

Note that a signal 1211, a signal 1212, a signal 1213, and a signal 1216shown in FIG. 12 are input to the first wiring 1111, the second wiring1112, the third wiring 1113, and the sixth wiring 1116, respectively.Here, each of the signal 1211, the signal 1212, the signal 1213, and thesignal 1216 is a digital signal in which a potential of an H-levelsignal is at V1 (hereinafter also referred to as an H level) and apotential of an L-level signal is at V2 (hereinafter also referred to asan L level). Further, the signal 1211, the signal 1212, the signal 1213,and the signal 1216 may be referred to as a start signal, a first clocksignal, a second clock signal (an inverted clock signal), and a resetsignal, respectively.

Note that any signal, potential, or current may be input to each of thefirst wiring 1111 to the sixth wiring 1116.

A digital signal in which a potential of an H-level signal is at V1(hereinafter also referred to as an H level) and a potential of anL-level signal is at V2 (hereinafter also referred to as an L level) isoutput from each of the seventh wirings 1117_1 to 1117_n. Note thatsince signals are output from the seventh wirings 1117_1 to 1117_nthrough a buffer 1401_1 to a buffer 1401_n, respectively, and an outputsignal of the shift register and a transfer signal of each flip-flop canbe divided, a range of operating conditions can be widened.

Here, examples of the buffer 1401_1 to the buffer 1401_n which areincluded in a shift register shown in FIG. 14 are described withreference to FIGS. 15A and 15B. In a buffer 8000 shown in FIG. 15A, aninverter 8001 a, an inverter 8001 b, and an inverter 8001 c areconnected between a wiring 8011 and a wiring 8012, and thus an invertedsignal of a signal which is input to the wiring 8011 is output from thewiring 8012. Note that the number of inverters which are connectedbetween the wiring 8011 and the wiring 8012 is not limited, and forexample, a signal having the same polarity as that of the signal whichis input to the wiring 8011 is output from the second wiring 8012 wheneven numbers of inverters are connected between the wiring 8011 and thewiring 8012. In addition, as shown in a buffer 8100 in FIG. 15B, aninverter 8002 a, an inverter 8002 b, and an inverter 8002 c which areconnected in series and an inverter 8003 a, an inverter 8003 b, and aninverter 8003 c which are provided in series may be connected inparallel. Since variation in characteristics of transistors can beaveraged in the buffer 8100 in FIG. 15B, delay and dullness of thesignal which is output from the wiring 8012 can be reduced. Further, theinverter 8002 a and output of the inverter 8003 a may be connected, andthe inverter 8002 b and output of the inverter 8003 b may be connected.

Note that in FIG. 15A, it is preferable to satisfy W of a transistorincluded in the inverter 8001 a<W of a transistor included in theinverter 8001 b<W of a transistor included in the inverter 8001 c. W ofthe transistor included in the inverter 8001 a is small and drivecapability (specifically, the value of W/L of the transistor in FIG. 1)of the flip-flop can be decreased, and thus a layout area in a shiftregister of the present invention can be reduced. Similarly, in FIG.15B, it is preferable to satisfy W of a transistor included in theinverter 8002 a<W of a transistor included in the inverter 8002 b<W of atransistor included in the inverter 8002 c. Similarly, in FIG. 15B, itis preferable to satisfy W of a transistor included in the inverter 8003a<W of a transistor included in the inverter 8003 b<W of a transistorincluded in the inverter 8003 c. Further, it is preferable to satisfy Wof the transistor included in the inverter 8002 a=W of the transistorincluded in the inverter 8003 a, W of the transistor included in theinverter 8002 b=W of the transistor included in the inverter 8003 b, andW of the transistor included in the inverter 8002 c=W of the transistorincluded in the inverter 8003 c.

Note that the inverters shown in FIGS. 15A and 15B are not particularlylimited as long as they can output inverted signals of input signals.For example, as shown in FIG. 15C, an inverter may be formed from afirst transistor 8201 and a second transistor 8202. In addition, asignal is input to a first wiring 8211; a signal is output from a secondwiring 8212; V1 is supplied to a third wiring 8213; and V2 is suppliedto a fourth wiring 8214. In the inverter in FIG. 15C, when an H-levelsignal is input to the first wiring 8211, a potential in which V1−V2 isdivided by the first transistor 8201 and the second transistor 8202 (W/Lof the first transistor 8201<W/L of the second transistor 8202) isoutput from the second wiring 8212. Further, in the inverter in FIG.15C, when an L-level signal is input to the first wiring 8211, V1−Vth8201 (Vth 8201 corresponds to the threshold voltage of the firsttransistor 8201) is output from the second wiring 8212. Furthermore, thefirst transistor 8201 may be a PN junction diode or simply a resistor aslong as it is an element having a resistance component.

In addition, as shown in FIG. 15D, an inverter may be formed from afirst transistor 8301, a second transistor 8302, a third transistor8303, and a fourth transistor 8304. Further, a signal is input to afirst wiring 8311; a signal is output from a second wiring 8312; V1 issupplied to a third wiring 8313 and a fifth wiring 8315; and V2 issupplied to a fourth wiring 8314 and a sixth wiring 8316. In theinverter in FIG. 15D, when an H-level signal is input to the firstwiring 8311, V2 is output from the second wiring 8312. At this time,since a potential of a node 8341 is at an L level, the first transistor8301 is turned off. Furthermore, in the inverter in FIG. 15D, when anL-level signal is input to the first wiring 8311, V1 is output from thesecond wiring 8312. At this time, when the potential of the node 8341becomes V1−Vth8303 (Vth8303 corresponds to the threshold voltage of thethird transistor 8303), the node 8341 enters into a floating state andthe potential of the node 8341 becomes higher than V1+Vth8301 (Vth8301corresponds to the threshold voltage of the first transistor 8301) by abootstrap operation, so that the first transistor 8301 is turned on.Moreover, since the first transistor 8301 functions as a bootstraptransistor, a capacitor may be provided between a second electrode and agate electrode of the first transistor 8301.

In addition, as shown in FIG. 16A, an inverter may be formed from afirst transistor 8401, a second transistor 8402, a third transistor8403, and a fourth transistor 8404. The inverter in FIG. 16A is atwo-input inverter and can perform a bootstrap operation. Further, asignal is input to a first wiring 8411; an inverted signal is input to asecond wiring 8412; a signal is output from a third wiring 8413; V1 issupplied to a fourth wiring 8414 and a sixth wiring 8416; and V2 issupplied to a fifth wiring 8415 and a seventh wiring 8417. In theinverter in FIG. 16A, when an L-level signal is input to the firstwiring 8411 and an H-level signal is input to the second wiring 8412, V2is output from the third wiring 8413. At this time, since a potential ofa node 8441 is at V2, the first transistor 8401 is turned off.Furthermore, in the inverter in FIG. 16A, when an H-level signal isinput to the first wiring 8411 and an L-level signal is input to thesecond wiring 8412, V1 is output from the third wiring 8413. At thistime, when the potential of the node 8441 becomes V1−Vth8403 (Vth8403corresponds to the threshold voltage of the third transistor 8403), thenode 8441 enters into a floating state and the potential of the node8441 becomes higher than V1+Vth8401 (Vth8401 corresponds to thethreshold voltage of the first transistor 8401) by a bootstrapoperation, so that the first transistor 8401 is turned on. Moreover,since the first transistor 8401 functions as a bootstrap transistor, acapacitor may be provided between a second electrode and a gateelectrode of the first transistor 8401. It is preferable that one of thefirst wiring 8411 and the second wiring 8412 be connected to the thirdwiring 123 shown in FIG. 1A and the other of the first wiring 8411 andthe second wiring 8412 be connected to the node 142 shown in FIG. 1A.

In addition, as shown in FIG. 16B, an inverter may be formed from afirst transistor 8501, a second transistor 8502, and a third transistor8503. The inverter in FIG. 16B is a two-input inverter and can perform abootstrap operation. Further, a signal is input to a first wiring 8511;an inverted signal is input to a second wiring 8512; a signal is outputfrom a third wiring 8513; V1 is supplied to a fourth wiring 8514 and asixth wiring 8516; and V2 is supplied to a fifth wiring 8515. In theinverter in FIG. 16B, when an L-level signal is input to the firstwiring 8511 and an H-level signal is input to the second wiring 8512, V2is output from the third wiring 8513. At this time, since a potential ofa node 8541 is at V2, the first transistor 8501 is turned off.Furthermore, in the inverter in FIG. 16B, when an H-level signal isinput to the first wiring 8511 and an L-level signal is input to thesecond wiring 8512, V1 is output from the third wiring 8513. At thistime, when the potential of the node 8541 becomes V1−Vth8503 (Vth8503corresponds to the threshold voltage of the third transistor 8503), thenode 8541 enters into a floating state and the potential of the node8541 becomes higher than V1+Vth8501 (Vth8501 corresponds to thethreshold voltage of the first transistor 8501) by a bootstrapoperation, so that the first transistor 8501 is turned on. Moreover,since the first transistor 8501 functions as a bootstrap transistor, acapacitor may be provided between a second electrode and a gateelectrode of the first transistor 8501. It is preferable that one of thefirst wiring 8511 and the second wiring 8512 be connected to the thirdwiring 123 shown in FIG. 1A and the other of the first wiring 8511 andthe second wiring 8512 be connected to the node 142 shown in FIG. 1A.

In addition, as shown in FIG. 16C, an inverter may be formed from afirst transistor 8601, a second transistor 8602, a third transistor8603, and a fourth transistor 8604. The inverter in FIG. 16C is atwo-input inverter and can perform a bootstrap operation. Further, asignal is input to a first wiring 8611; an inverted signal is input to asecond wiring 8612; a signal is output from a third wiring 8613; V1 issupplied to a fourth wiring 8614; and V2 is supplied to a fifth wiring8615 and a sixth wiring 8616. In the inverter in FIG. 16C, when anL-level signal is input to the first wiring 8611 and an H-level signalis input to the second wiring 8612, V2 is output from the third wiring8613. At this time, since a potential of a node 8641 is at V2, the firsttransistor 8601 is turned off. Furthermore, in the inverter in FIG. 16C,when an H-level signal is input to the first wiring 8611 and an L-levelsignal is input to the second wiring 8612, V1 is output from the thirdwiring 8613. At this time, when the potential of the node 8641 becomesV1−Vth8603 (Vth8603 corresponds to the threshold voltage of the thirdtransistor 8603), the node 8641 enters into a floating state and thepotential of the node 8641 becomes higher than V1+Vth8601 (Vth8601corresponds to the threshold voltage of the first transistor 8601) by abootstrap operation, so that the first transistor 8601 is turned on.Moreover, since the first transistor 8601 functions as a bootstraptransistor, a capacitor may be provided between a second electrode and agate electrode of the first transistor 8601. It is preferable that oneof the first wiring 8611 and the second wiring 8612 be connected to thethird wiring 123 shown in FIG. 1A and the other of the first wiring 8611and the second wiring 8612 be connected to the node 142 shown in FIG.1A.

Note that a signal output from the seventh wiring 1117_i−1 is used as astart signal of the flip-flop 1101_i, and a signal output from theseventh wiring 1117_i+1 is used as a reset signal of the flip-flop 1101.A start signal of the flip-flop 1101_1 is input from the first wiring1111, and a reset signal of the flip-flop 1101_n is input from the sixthwiring 1116. Note also that as the reset signal of the flip-flop 1101_n,a signal output from the seventh wiring 1117_1 or a signal output fromthe seventh wiring 1117_2 may be used. Alternatively, a dummy flip-flopmay be additionally provided and an output signal of the dummy flip-flopmay be used. Thus, the number of the wirings and the number of thesignals can be reduced.

As shown in FIG. 13, for example, when the flip-flop 1101_i enters theselection period, an H-level signal (a selection signal) is output fromthe seventh wiring 1117_i. At this time, the flip-flop 1101_i+1 entersthe set period. After that, the flip-flop 1101_i enters the reset periodand an L-level signal is output from the seventh wiring 1117_i. At thistime, the flip-flop 1101_i+1 enters the selection period. After that,the flip-flop 1101_i enters the first non-selection period, and theseventh wiring 1117_i enters into a floating state and remains at V2. Atthis time, the flip-flop 1101_i+1 enters the reset period. After that,the flip-flop 1101_i enters the second non-selection period and anL-level signal is output from the seventh wiring 1117_i. At this time,the flip-flop 1101_i+1 enters the first non-selection period.

In the shift register in FIG. 11, the selection signal can be outputsequentially from the seventh wiring 1117_1 to the seventh wiring 1117_nin this manner. That is, in the shift register in FIG. 11, the seventhwiring 1117_1 to the seventh wiring 1117_n can be scanned.

In addition, since the threshold voltage shift of each transistor can besuppressed in a shift register to which the flip-flop of this embodimentmode is applied, the life can be prolonged. In addition, since thresholdvoltage shifts of all the transistors can be suppressed in the flip-flopof this embodiment mode, the life can be prolonged. Further, in theshift register to which the flip-flop of this embodiment mode isapplied, reliability can be improved. Furthermore, in the shift registerto which the flip-flop of this embodiment mode is applied, a malfunctioncan be suppressed.

In addition, since the shift register to which the flip-flop of thisembodiment mode is applied can operate at high speed, it can be appliedto a higher-definition display device or a larger display device.Further, in the shift register to which the flip-flop of this embodimentmode is applied, a process can be simplified. Furthermore, in the shiftregister to which the flip-flop of this embodiment mode is applied,manufacturing cost can be reduced. Moreover, in the shift register towhich the flip-flop of this embodiment mode is applied, yield can beimproved.

Next, a structure and a driving method of a display device including theabove-described shift register of this embodiment mode are described.Note that it is only necessary that the display device of thisembodiment mode at least include the flip-flop of this embodiment mode.

The structure of the display device of this embodiment mode is describedwith reference to FIG. 17. The display device in FIG. 17 includes asignal line driver circuit 1701, a scan line driver circuit 1702, and apixel portion 1704. The pixel portion 1704 includes a plurality ofsignal lines S1 to Sm extended from the signal line driver circuit 1701in a column direction, a plurality of scan lines G1 to Gn extended fromthe scan line driver circuit 1702 in a row direction, and a plurality ofpixels 1703 arranged in matrix in accordance with the signal lines S1 toSm and the scan lines G1 to Gn. In addition, each of the pixels 1703 isconnected to a signal line Sj (any one of the signal lines S1 to Sm) anda scan line Gi (any one of the scan lines G1 to Gn). Further, the scanline driver circuit 1702 may be referred to as a driver circuit.

Note that the shift register of this embodiment mode can be used as thescan line driver circuit 1702. Needless to say, the shift register ofthis embodiment mode may be used as the signal line driver circuit 1701.

Note that the scan lines G1 to Gn are connected to the seventh wirings1117_1 to 1117_n.

Note also that each of the signal lines and the scan lines may be simplyreferred to as a wiring. In addition, each of the signal line drivercircuit 1701 and the scan line driver circuit 1702 may be referred to asa driver circuit.

Each of the pixels 1703 at least includes a switching element, acapacitor, and a pixel electrode. Note that each of the pixels 1703 mayinclude a plurality of switching elements or a plurality of capacitors.In addition, each of the pixels 1703 does not necessarily include acapacitor. Further, each of the pixels 1703 may further include atransistor which operates in a saturation region. Furthermore, each ofthe pixels 1703 may include a display element such as a liquid crystalelement or an EL element. Here, a transistor or a PN junction diode canbe used as a switching element. Note also that when a transistor is usedas the switching element, it is preferable that the transistor operatein a linear region. In addition, when the scan line driver circuit 1702is formed by using only N-channel transistors, it is preferable that anN-channel transistor be used as the switching element. Alternatively,when the scan line driver circuit 1702 is formed by using only P-channeltransistors, it is preferable that a P-channel transistor be used as theswitching element.

The scan line driver circuit 1702 and the pixel portion 1704 are formedover an insulating substrate 1705, and the signal line driver circuit1701 is not formed over the insulating substrate 1705. The signal linedriver circuit 1701 is formed using a single crystalline substrate, anSOI substrate, or an insulating substrate, which is different from theinsulating substrate 1705. In addition, the signal line driver circuit1701 is connected to the signal lines S1 to Sm through a printed circuitsuch as an FPC. Note that the signal line driver circuit 1701 may beformed over the insulating substrate 1705, or a circuit which forms partof the signal line driver circuit 1701 may be formed over the insulatingsubstrate 1705.

Note that the above-described wirings and/or the electrodes can also beapplied to other display devices, shift registers, and pixels.

The signal line driver circuit 1701 inputs voltage or current as a videosignal to each of the signal lines S1 to Sm. Note that the video signalmay be either a digital signal or an analog signal. In addition, apositive electrode and a negative electrode of the video signal may beinverted in each frame (i.e., frame inversion driving), may be invertedin each row (i.e., gate line inversion driving), may be inverted in eachcolumn (i.e., source line inversion driving), or may be inverted in eachrow and each column (i.e., dot inversion driving). Further, the videosignal may be input to each of the signal lines S1 to Sm by dotsequential driving or line sequential driving. Furthermore, the signalline driver circuit 1701 may input not only the video signal but alsoconstant voltage such as precharge voltage to each of the signal linesS1 to Sm. It is preferable that a constant voltage such as prechargevoltage be input in each gate selection period or each frame.

Note that the scan line driver circuit 1702 inputs a signal to each ofthe scan line G1 to Gn and sequentially selects (hereinafter alsoreferred to as scans) the scan lines G1 to Gn from a first row. Then,the scan line driver circuit 1702 selects a plurality of the pixels 1703connected to the selected scan lines. Here, a period in which one scanline is selected is referred to as one gate selection period and aperiod in which one scan line is not selected is referred to as anon-selection period. In addition, the signal which is output to eachscan line by the scan line driver circuit 1702 is referred to as a scansignal. Further, the maximum value of the scan signal is higher than themaximum value of the video signal or the maximum voltage of the signalline, and the minimum value of the scan signal is lower than the minimumvalue of the video signal or the minimum voltage of the signal line.

When the pixel 1703 is selected, a video signal is input to the pixel1703 from the signal line driver circuit 1701 through the signal lineAlternatively, when the pixel 1703 is not selected, the pixel 1703 holdsa video signal (a potential in accordance with the video signal) whichis input in the selection period.

Although not shown, a plurality of potentials and a plurality of signalsare supplied to each of the signal line driver circuit 1701 and the scanline driver circuit 1702.

Next, operations of the display device shown in FIG. 17 are describedwith reference to a timing chart in FIG. 18. Note that FIG. 18 shows oneframe period which corresponds to a period for displaying an image forone screen. Note that although one frame period is not particularlylimited, it is preferable that one frame period be 1/60 second or lessso that a person viewing an image does not perceive a flicker.

Note that the timing chart in FIG. 18 shows selection timing of each ofthe scan line G1 of a first row, the scan line Gi of an i-th row, thescan line Gi+1 of an (i+1)th row, and the scan line Gn of an n-th row.

In FIG. 18, for example, the scan line Gi of the i-th row is selectedand a plurality of the pixels 1703 connected to the scan line Gi areselected. Then, a video signal is input to each of the plurality of thepixels 1703 connected to the scan line Gi and each of the plurality ofthe pixels 1703 connected to the scan line Gi holds a potential inaccordance with the video signal. After that, the scan line Gi of thei-th row is not selected, the scan line Gi+1 of the (i+1)th row isselected, and a plurality of the pixels 1703 connected to the scan lineGi+1 are selected. Then, a video signal is input to each of theplurality of the pixels 1703 connected to the scan line Gi+1 and each ofthe plurality of the pixels 1703 connected to the scan line Gi+1 holds apotential in accordance with the video signal. The scan lines G1 to Gnare sequentially selected in one frame period in this manner, and theplurality of the pixels 1703 connected to each scan line aresequentially selected. Then, a video signal is input to each of theplurality of the pixels 1703 connected to each scan line and each of theplurality of the pixels 1703 connected to each scan line holds apotential in accordance with the video signal.

In addition, since a display device using the shift register of thisembodiment mode as the scan line driver circuit 1702 can operate at highspeed, the display device can be made larger or can be made higherdefinition. Further, in the display device of this embodiment mode, aprocess can be simplified. Furthermore, in the display device of thisembodiment mode, manufacturing cost can be reduced. Moreover, in thedisplay device of this embodiment mode, yield can be improved.

Note that in the display device in FIG. 17, since the signal line drivercircuit 1701 which necessarily operates at high speed, and the scan linedriver circuit 1702 and the pixel portion 1704 are formed over differentsubstrates, amorphous silicon can be used for a semiconductor layer of atransistor included in the scan line driver circuit 1702 and asemiconductor layer of a transistor included in the pixel 1703.Therefore, in the display device in FIG. 17, the manufacturing processcan be simplified. In addition, in the display device in FIG. 17,manufacturing cost can be reduced. Further, in the display device inFIG. 17, yield can be improved. Furthermore, the display device in FIG.17 can be made larger. Alternatively, even when polysilicon or singlecrystalline silicon is used for the semiconductor layer of eachtransistor, the manufacturing process can be simplified.

When the signal line driver circuit 1701, and the scan line drivercircuit 1702 and the pixel 1703 are formed over the same substrate, itis preferable that polysilicon or a single crystalline silicon be usedfor the semiconductor layer of the transistor included in the scan linedriver circuit 1702 and the semiconductor layer of the transistorincluded in the pixel 1703.

Note that the number, arrangement, and the like of each driver circuitare not limited to those of FIG. 17 as long as a pixel is selected and avideo signal can be written to the pixel as shown in FIG. 17.

For example, as shown in FIG. 19, the scan lines G1 to Gn may be scannedwith a first scan line driver circuit 1902 a and a second scan linedriver circuit 1902 b. Each of the first scan line driver circuit 1902 aand the second scan line driver circuit 19026 has a structure which issimilar to that of the scan line driver circuit 1702 shown in FIG. 17;corresponding wirings are electrically connected with each other in thefirst scan line driver circuit 1902 a and the second scan line drivercircuit 1902 b; the first scan line driver circuit 1902 a and the secondscan line driver circuit 1902 b scan the scan lines G1 to Gn with thesame timing. Further, the first scan line driver circuit 1902 a and thesecond scan line driver circuit 1902 b may be referred to as a firstdriver circuit and a second driver circuit, respectively.

Even when a defect occurs in one of the first scan line driver circuit1902 a and the second scan line driver circuit 1902 b in a displaydevice in FIG. 19, the scan lines G1 to Gn can be scanned with the otherof the first scan line driver circuit 1902 a and the second scan linedriver circuit 1902 b. Therefore, the display device in FIG. 19 can haveredundancy. In addition, a load (wiring resistance and parasiticcapacitance of the scan lines) of the first scan line driver circuit1902 a and a load of the second scan line driver circuit 1902 b in thedisplay device in FIG. 19 can be reduced approximately in half of thosein the display device in FIG. 17. Therefore, delay and dullness ofsignals input to the scan lines G1 to Gn (output signals of the firstscan line driver circuit 1902 a and the second scan line driver circuit1902 b) can be reduced. Further, since the load of the first scan linedriver circuit 1902 a and the load of the second scan line drivercircuit 1902 b in the display device in FIG. 19 can be reduced, the scanlines G1 to Gn can be scanned at high speed. Furthermore, since the scanlines G1 to Gn can be scanned at high speed, a panel can be made largeror can be made higher definition. Note that portions which are common tothose in FIG. 17 are denoted by common reference numerals anddescription thereof is omitted.

As another example, FIG. 20 shows a display device in which a videosignal can be written to a pixel at high speed. In the display device inFIG. 20, a video signal is input to the pixels 1703 of odd-numbered rowsfrom signal lines of odd-numbered columns, and a video signal is inputto the pixels 1703 of even-numbered rows from signal lines ofeven-numbered columns. In addition, in the display device in FIG. 20,scan lines of odd-numbered stages among the scan lines G1 to Gn arescanned with a first scan line driver circuit 2002 a, and scan lines ofeven-numbered stages among the scan lines G1 to Gn are scanned with asecond scan line driver circuit 2002 b. Further, a start signal input tothe first scan line driver circuit 2002 a is input later than a startsignal input to the second scan line driver circuit 2002 b for a ¼period of a clock signal.

Note that in the display device in FIG. 20, dot inversion driving can beperformed just by inputting a positive video signal and a negative videosignal to each signal line in each column in one frame period. Inaddition, in the display device in FIG. 20, frame inversion driving canbe performed by inverting polarity of a video signal input to eachsignal line in each one frame period.

Operations of the display device in FIG. 20 are described with referenceto a timing chart in FIG. 21. Note that the timing chart in FIG. 21shows selection timing of each of the scan line G1 of first row, thescan line Gi−1 of an (i−1)th row, the scan line Gi of the i-th row, thescan line Gi+1 of the (i+1)th row, and the scan line Gn of the n-th row.In addition, in the timing chart in FIG. 21, one selection period isdivided into a selection period a and a selection period b. Further, thetiming chart in FIG. 21 shows the case where dot inversion driving andframe inversion driving are performed in the display device in FIG. 20.

In FIG. 21, for example, the selection period a of the scan line Gi ofthe i-th row overlaps with the selection period b of the scan line Gi−1of the (i−1)th row, and the selection period b of the scan line Gi ofthe i-th row overlaps with the selection period a of the scan line Gi+1of the (i+1)th row. Therefore, in the selection period a, a video signalwhich is similar to a video signal input to the pixel 1703 of the(i−1)th row and a (j+1)th column is input to the pixel 1703 of the i-throw and the j-th column. In addition, in the selection period b, a videosignal which is similar to the video signal input to the pixel 1703 ofthe i-th row and the j-th column is input to the pixel 1703 of the(i+1)th row and the (j+1)th column. Note that the video signal input toeach of the pixels 1703 in the selection period b is an original videosignal, and the video signal input to each of the pixels 1703 in theselection period a is a precharge video signal of each of the pixels1703. Therefore, after each of the pixels 1703 is precharged by thevideo signal input to the pixel 1703 of the (i−1)th row and (j+1)thcolumn in the selection period a, the original video signal (of the i-throw and j-th column) is input to each of the pixels 1703 in theselection period b.

As described above, since the video signal can be written to each of thepixels 1703 at high speed, the display device in FIG. 20 can be easilymade larger or can be easily made higher definition. In addition, sincea video signal having the same polarity is input to each signal line inone frame period, there is not much charging and discharging of eachsignal line and low power consumption can be achieved. Further, since aload of an IC for inputting the video signal can be significantlyreduced in the display device in FIG. 20, heat generation, powerconsumption, and the like of the IC can be reduced. Furthermore, sincedrive frequency of the first scan line driver circuit 2002 a and thesecond scan line driver circuit 2002 b can be reduced approximately inhalf in the display device in FIG. 20, power can be saved.

Note that in the display device of this embodiment mode, various drivingmethods can be performed depending on the structure and the drivingmethod of the pixels 1703. For example, the scan lines may be scannedwith the scan line driver circuits a plurality of times in one frameperiod.

Note that another wiring or the like may be added to each of the displaydevices in FIGS. 17, 19, and 20 depending on the structure of the pixels1703. For example, a constant power supply line, a capacitor line, ascan line, or the like may be added. Note also that in the case ofadding a scan line, a scan line driver circuit to which the shiftregister of this embodiment mode is applied may be added. As anotherexample, a dummy scan line, a signal line, a power supply line, or acapacitor line may be provided to the pixel portion.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 2

In this embodiment mode, structures and driving methods of a flip-flopwhich is different from those of Embodiment Mode 1, a driver circuitincluding the flip-flop, and a display device including the drivercircuit are described. Note that portions which are similar to those ofEmbodiment Mode 1 are denoted by common reference numerals and detaileddescription of the portions which are the same and portions which havesimilar functions is omitted.

As a structure of the flip-flop of this embodiment mode, a structurewhich is similar to that of the flip-flop of Embodiment Mode 1 can beused. Note that drive timing of the flip-flop is different from that ofEmbodiment Mode 1. Thus, in this embodiment mode, description of thestructure of the flip-flop is omitted.

Note that although the case is described in which the drive timing ofthis embodiment mode is applied to the flip-flop in FIG. 1A, the drivetiming of this embodiment mode can be freely combined with each of theflip-flops in FIGS. 1B, 1C, 5A, 5B, 5C, 7A, 7B, 8A, 8B, 9A, 9B, 10A, and10B. In addition, the drive timing of this embodiment mode can be freelycombined with the drive timing described in Embodiment Mode 1.

Next, operations of the flip-flop of this embodiment mode are describedwith reference to the flip-flop in FIG. 1A and a timing chart in FIG.22. Note that the timing chart in FIG. 22 is described by dividing thewhole period into a set period, a selection period, a reset period, afirst non-selection period, and a second non-selection period. Note alsothat the set period is divided into a first set period and a second setperiod, and the selection period is divided into a first selectionperiod and a second selection period.

Note that a signal 2221, a signal 2225, a signal 2228, a signal 2227,and a signal 2222 shown in FIG. 22 are input to the first wiring 121,the fifth wiring 125, the eighth wiring 128, the seventh wiring 127, andthe second wiring 122, respectively. In addition, a signal 2223 shown inFIG. 22 is output from the third wiring 123. Here, each of the signal2221, the signal 2225, the signal 2228, the signal 2227, the signal2222, and the signal 2223 is a digital signal in which a potential of anH-level signal is at V1 (hereinafter also referred to as an H level) anda potential of an L-level signal is at V2 (hereinafter also referred toas an L level). Further, the signal 2221, the signal 2225, the signal2228, the signal 2227, the signal 2222, and the signal 2223 may bereferred to as a start signal, a power clock signal (PCK), a firstcontrol clock signal (CCK1), a second control clock signal (CCK2), areset signal, and an output signal, respectively.

The flip-flop of this embodiment mode basically performs operationswhich are similar to those of the flip-flop described in EmbodimentMode 1. Note that in the flip-flop of this embodiment mode, timing atwhich an H-level signal is input to the first wiring 121 is delayed fora ¼ period of a clock signal, which is different from the flip-flop ofEmbodiment Mode 1.

In a first set period (A1), a second set period (A2), a reset period(C), a first non-selection period (D), and a second non-selection period(E) shown in FIG. 22, the flip-flop of this embodiment mode performsoperations which are similar to those in the second non-selection period(E), the set period (A), the reset period (C), the first non-selectionperiod (D), and the second non-selection period (E) shown in FIG. 2.Thus, description thereof is omitted.

Note that as shown in FIG. 23, by delaying timing for inputting anH-level signal to the second wiring 122 for a ¼ period of a clocksignal, fall time of an output signal can be significantly shortened.That is, in the flip-flop of this embodiment mode to which FIG. 23 isapplied, an L-level signal is input to the fifth wiring in a first resetperiod shown in period C1 of FIG. 23 and a potential of the node 141lowers to approximately V1+Vth101. Therefore, the first transistor 101remains on and an L-level signal is output from the third wiring 123.Since an L-level signal is input to the third wiring 123 through thefirst transistor 101 having a larger value of W/L, time when a potentialof the third wiring 123 becomes an L level from an H level can besignificantly shortened. After that, in the flip-flop of this embodimentmode to which FIG. 23 is applied, the seventh transistor 107 is turnedon in a second reset period shown in period C2 of FIG. 23 and thepotential of the node 141 becomes V2. Since a potential of the node 142at this time becomes V1−Vth103 and the third transistor 103 is turnedon, an L-level signal is output from the third wiring 123.

In the flip-flop of this embodiment mode, advantageous effects which aresimilar to those of the flip-flop shown in Embodiment Mode 1 can beobtained.

Next, a structure and a driving method of a shift register including theabove-described flip-flop of this embodiment mode are described.

The structure of the shift register of this embodiment mode is describedwith reference to FIG. 24. The shift register in FIG. 24 includes npieces of flip-flops (flip-flops 2401_1 to 2401_n).

Connection relations of the shift register in FIG. 24 are described. Ina flip-flop 2401_i of an i-th stage (any one of the flip-flops 2401_1 to2401_n) of the shift register in FIG. 24, the first wiring 121 shown inFIG. 1A is connected to a tenth wiring 2420_i−1; the second wiring 122shown in FIG. 1A is connected to a tenth wiring 2420_i+2; the thirdwiring 123 shown in FIG. 1A is connected to a tenth wiring 2420_i; thefourth wiring 124, the tenth wiring 130, the eleventh wiring 131, thetwelfth wiring 132, and the thirteenth wiring 133 shown in FIG. 1A areconnected to a seventh wiring 2417; the fifth wiring 125 and the seventhwiring 127 shown in FIG. 1A are connected to a second wiring 2412 in aflip-flop of a (4N−3)th stage (N corresponds to a natural number whichis 1 or more); the fifth wiring 125 and the seventh wiring 127 shown inFIG. 1A are connected to a third wiring 2413 in a flip-flop of a(4N−2)th stage; the fifth wiring 125 and the seventh wiring 127 shown inFIG. 1A are connected to a fourth wiring 2414 in a flip-flop of a(4N−1)th stage; the fifth wiring 125 and the seventh wiring 127 shown inFIG. 1A are connected to a fifth wiring 2415 in a flip-flop of a 4N-thstage; the eighth wiring 128 shown in FIG. 1A is connected to the fourthwiring 2413 in the flip-flop of the (4N−3)th stage; the eighth wiring128 shown in FIG. 1A is connected to the fifth wiring 2415 in theflip-flop of the (4N−2)th stage; the eighth wiring 128 shown in FIG. 1Ais connected to the second wiring 2412 in the flip-flop of the (4N−1)thstage; the eighth wiring 128 shown in FIG. 1A is connected to the thirdwiring 2413 in the flip-flop of the 4N-th stage; and the sixth wiring126 and the ninth wiring 129 shown in FIG. 1A are connected to a sixthwiring 2416. Note that the first wiring 121 shown in FIG. 1A of theflip-flop 2401_1 of a first stage is connected to a first wiring 2411;the second wiring 122 shown in FIG. 1A of the flip-flop 2401_n−1 of an(n−1)th stage is connected to a ninth wiring 2419; and the second wiring122 shown in FIG. 1A of the flip-flop 2401_n of an n-th stage isconnected to an eighth wiring 2418.

Note that when the timing chart in FIG. 23 is applied to the flip-flopof this embodiment mode, the second wiring 122 shown in FIG. 1A of theflip-flop 2401_i of the i-th stage is connected to a tenth wiring2420_i+3. Therefore, the second wiring 122 shown in FIG. 1A of theflip-flop 2401_n−3 of an (n−3)th stage is connected to a wiring which isadditionally provided.

Note also that the first wiring 2411, the second wiring 2412, the thirdwiring 2413, the fourth wiring 2414, the fifth wiring 2415, the eighthwiring 2418, and the ninth wiring 2419 may be referred to as a firstsignal line, a second signal line, a third signal line, a fourth signalline, a fifth signal line, a sixth signal line, and a seventh signalline, respectively. Further, the sixth wiring 2416 and the seventhwiring 2417 may be referred to as a first power supply line and a secondpower supply line, respectively.

Next, operations of the shift register shown in FIG. 24 are describedwith reference to a timing chart in FIG. 25 and a timing chart in FIG.26. Here, the timing chart in FIG. 25 is divided into a scanninginterval and a retrace interval.

Note that the potential of V1 is supplied to the fourth wiring 2414 andthe potential of V2 is supplied to the fifth wiring 2415.

Note that a signal 2511, a signal 2512, a signal 2513, a signal 2514, asignal 2515, a signal 2518, and a signal 2519 shown in FIG. 25 are inputto the first wiring 2411, the second wiring 2412, the third wiring 2413,the fourth wiring 2414, the fifth wiring 2415, the eighth wiring 2418,and the ninth wiring 2419, respectively. Here, each of the signal 2511,the signal 2512, the signal 2513, the signal 2514, the signal 2515, thesignal 2518, and the signal 2519 is a digital signal in which apotential of an H-level signal is at V1 (hereinafter also referred to asan H level) and a potential of an L-level signal is at V2 (hereinafteralso referred to as an L level). Further, the signal 2511, the signal2512, the signal 2513, the signal 2514, the signal 2515, the signal2518, and the signal 2519 may be referred to as a start signal, a firstclock signal, a second clock signal, a third clock signal, a fourthclock signal, a first reset signal, and a second reset signal,respectively.

Note that any signal, potential, or current may be input to each of thefirst wiring 2411 to the ninth wiring 2419.

A digital signal in which a potential of an H-level signal is at V1(hereinafter also referred to as an H level) and a potential of anL-level signal is at V2 (hereinafter also referred to as an L level) isoutput from each of the tenth wirings 2420_1 to 2420_n. Note that byconnecting a buffer to each of the tenth wirings 2420_1 to 2420_nsimilarly to Embodiment Mode 1, a range of operating conditions can bewidened.

Note that a signal output from the tenth wiring 2420_i−1 is used as astart signal of the flip-flop 2401_i, and a signal output from the tenthwiring 2420_i+2 is used as a reset signal of the flip-flop 2401_i. Here,a start signal of the flip-flop 2401_1 is input from the first wiring2411; a second reset signal of the flip-flop 2401_n−1 is input from theninth wiring 2419; and a first reset signal of the flip-flop 2401_n isinput from the eighth wiring 2418. Note also that a signal output fromthe tenth wiring 2420_1 may be used as the second reset signal of theflip-flop 2401_n−1, and a signal output from the tenth wiring 2420_2 maybe used as the first reset signal of the flip-flop 2401_n.Alternatively, a signal output from the tenth wiring 2420_2 may be usedas the second reset signal of the flip-flop 2401_n−1, and a signaloutput from the tenth wiring 2420_3 may be used as the first resetsignal of the flip-flop 2401_n. Further alternatively, a first dummyflip-flop and a second dummy flip-flop may be additionally provided, andan output signal of the first dummy flip-flop and an output signal ofthe second dummy flip-flop may be used as the first reset signal and thesecond reset signal, respectively. Thus, the number of the wirings andthe number of the signals can be reduced.

As shown in FIG. 26, for example, when the flip-flop 2401_i enters thefirst selection period, an H-level signal (a selection signal) is outputfrom the tenth wiring 2420_i. At this time, the flip-flop 2401_i+1enters the second set period. After that, when the flip-flop 2401_ienters the second selection period, the tenth wiring 2420_i keepsoutputting an H-level signal. At this time, the flip-flop 2401_41 entersthe first selection period. After that, when the flip-flop 2401_i entersthe reset period, an L-level signal is output from the tenth wiring2420_i. At this time, the flip-flop 2401_i+1 enters the second selectionperiod. After that, when the flip-flop 2401_i enters the firstnon-selection period, the tenth wiring 2420_i enters into a floatingstate and remains at V2. At this time, the flip-flop 2401_i+1 enters thereset period. After that, when the flip-flop 2401_i enters the secondnon-selection period, an L-level signal is output from the tenth wiring2420_i. At this time, the flip-flop 2401_i+1 enters the secondnon-selection period.

In the shift register in FIG. 24, the selection signal can be outputsequentially from the tenth wiring 2420_1 to the tenth wiring 2420_n inthis manner. Further, since the second selection period of the flip-flop2401_i and the first selection period of the flip-flop 2401_i+1 are thesame period, the selection signal can be output from the tenth wiring2420_i and the tenth wiring 2420_i+1 in the same period.

As described above, the shift register of this embodiment mode can beapplied to a higher-definition display device or a large display device.Further, in the shift register of this embodiment mode, advantageouseffects which are similar to those of the shift register shown inEmbodiment Mode 1 can be obtained.

Next, a structure and a driving method of a display device including theabove-described shift register of this embodiment mode are described.Note that it is only necessary that the display device of thisembodiment mode at least include the flip-flop of this embodiment mode.

The structure of the display device of this embodiment mode is describedwith reference to FIG. 27. In the display device in FIG. 27, the scanlines G1 to Gn are scanned with a scan line driver circuit 2702. Inaddition, a video signal is input to the pixels 1703 of odd-numberedrows from signal lines of odd-numbered columns, and a video signal isinput to the pixels 1703 of even-numbered rows from signal lines ofeven-numbered columns. Note that portions which are common to those inFIG. 17 are denoted by common reference numerals and description thereofis omitted.

Note that by applying the shift register of this embodiment mode to thescan line driver circuit 2702 in the display device in FIG. 27,operations which are similar to those of the display device in FIG. 20can be performed by one scan line driver circuit. Therefore,advantageous effects which are similar to those of the display device inFIG. 20 can be obtained.

Note also that similarly to the display device in FIG. 19, the scanlines G1 to Gn may be scanned with a first scan line driver circuit 2802a and a second scan line driver circuit 2802 b. Therefore, advantageouseffects which are similar to those of the display device in FIG. 19 canbe obtained. A structure of that case is shown in FIG. 28.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 3

In this embodiment mode, structures and driving methods of a flip-flopwhich is different from those of Embodiment Modes 1 and 2, a drivercircuit including the flip-flop, and a display device including thedriver circuit are described. In the flip-flop of this embodiment mode,an output signal of the flip-flop and a transfer signal of the flip-flopare output from different wirings by different transistors. Note thatportions which are similar to those of Embodiment Modes 1 and 2 aredenoted by common reference numerals and detailed description of theportions which are the same and portions which have similar functions isomitted.

A basic structure of the flip-flop of this embodiment mode is describedwith reference to FIG. 40. A flip-flop shown in FIG. 40 is similar tothe flip-flop in FIG. 1A to which a ninth transistor 109 and a tenthtransistor 110 are added.

Connection relations of the flip-flop in FIG. 40 are described. A firstelectrode of the ninth transistor 109 is connected to a fifteenth wiring135; a second electrode of the ninth transistor 109 is connected to afourteenth wiring 134; and a gate electrode of the ninth transistor 109is connected to the node 141. A first electrode of the tenth transistor110 is connected to a sixteenth wiring 136; a second electrode of thetenth transistor 110 is connected to the fourteenth wiring 134; and agate electrode of the tenth transistor 110 is connected to the eighthwiring 128. Other connection relations are similar to those of FIG. 1A.

Note that the fifteenth wiring 135 and the sixteenth wiring 136 may bereferred to as an eighth signal line and an eighth power supply line,respectively.

Next, operations of the flip-flop shown in FIG. 40 are described withreference to a timing chart shown in FIG. 41. Note that the timing chartin FIG. 41 is described by dividing the whole period into a set period,a selection period, a reset period, a first non-selection period, and asecond non-selection period. Note also that the set period, the resetperiod, the first non-selection period, and the second non-selectionperiod are collectively referred to as a non-selection period in somecases.

Note that the signal 223 and a signal 234 are output from the thirdwiring 123 and the fourteenth wiring 134, respectively. The signal 234is an output signal of the flip-flop and the signal 223 is a transfersignal of the flip-flop. Note also that the signal 223 may be the outputsignal of the flip-flop and the signal 234 may be the transfer signal ofthe flip-flop.

Therefore, when the signal 234 is used as the output signal of theflip-flop and the signal 223 is used as the transfer signal of theflip-flop, it is preferable that the ninth transistor 109 have thelargest value of W/L among the first transistor 101 to the tenthtransistor 110. Note that when the signal 223 is used as the outputsignal of the flip-flop and the signal 234 is used as the transfersignal of the flip-flop, it is preferable that the first transistor 101have the largest value of W/L among the first transistor 101 to thetenth transistor 110.

As described above, the output signal of the flip-flop and the transfersignal of the flip-flop are output from different wirings by differenttransistors in this embodiment mode. That is, in the flip-flop in FIG.40, a signal is output from the third wiring 123 by the first transistor101 and the second transistor 102, and a signal is output from thefourteenth wiring 134 by the ninth transistor 109 and the tenthtransistor 110. Further, since the ninth transistor 109 and the tenthtransistor 110 are connected similarly to the first transistor 101 andthe second transistor 102, a signal output from the fourteenth wiring134 (the signal 234) has a waveform which is almost the same as that ofa signal output from the third wiring 123 (the signal 223).

Note that since it is only necessary that the first transistor 101 cansupply a charge to the gate electrode of the fifth transistor 105 of thenext stage, the value of W/L of the first transistor 101 is preferablyless than or equal to twice, more preferably, less than or equal to thevalue of W/L of the fifth transistor 105.

Note also that the ninth transistor 109 and the tenth transistor 110have functions which are similar to those of the first transistor 101and the second transistor 102, respectively. Further, the ninthtransistor 109 and the tenth transistor 110 may be referred to as abuffer portion.

As described above, the flip-flop in FIG. 40 can prevent a malfunctioneven when a large load is connected to the fourteenth wiring 134 anddelay, dullness, or the like occurs in the signal 234. This is becausethe flip-flop in FIG. 40 is not adversely affected by delay, dullness,or the like of the output signal by outputting the output signal of theflip-flop and the transfer signal of the flip-flop from differentwirings by different transistors.

Further, in the flip-flop of this embodiment mode, advantageous effectswhich are similar to those of the flip-flops described in EmbodimentModes 1 and 2 can be obtained.

Note that the flip-flop of this embodiment mode can be freely combinedwith each of the flip-flops in FIGS. 1B, 1C, 5A, 5B, 5C, 7A, 7B, 8A, 8B,9A, 9B, 10A, and 10B. In addition, the flip-flop of this embodiment modecan be freely combined with the drive timings described in EmbodimentModes 1 and 2.

Next, a structure and a driving method of a shift register including theabove-described flip-flop of this embodiment mode are described.

The structure of the shift register of this embodiment mode is describedwith reference to FIG. 42. The shift register in FIG. 42 includes npieces of flip-flops (flip-flops 4201_1 to 4201_n).

The flip-flops 4201_1 to 4201_n, a first wiring 4211, a second wiring4212, a third wiring 4213, a fourth wiring 4214, a fifth wiring 4215,and a sixth wiring 4216 correspond to the flip-flops 1101_1 to 1101_n,the first wiring 1111, the second wiring 1112, the third wiring 1113,the fourth wiring 1114, the fifth wiring 1115, the sixth wiring 1116,respectively, and a similar signal or similar power supply voltage isinput thereto. In addition, seventh wirings 4217_1 to 4217_n and eighthwirings 4218_1 to 4218_n correspond to the seventh wirings 1117_1 to1117_n in FIG. 11.

Next, operations of the shift register shown in FIG. 42 are describedwith reference to a timing chart in FIG. 43.

The operations of the shift register shown in FIG. 42 are different fromthose of the shift register shown in FIG. 11 in that an output signaland a transfer signal are output to different wirings. Specifically, theoutput signal is output to each of the eighth wirings 4218_1 to 4218_n,and the transfer signal is output to each of the seventh wirings 4217_1to 4217_n.

Even when a large load (e.g., a resistor or a capacitor) is connected toeach of the eighth wirings 4218_1 to 4218_n, the shift register in FIG.42 can operate without being adversely affected by the load. Inaddition, the shift register in FIG. 42 can continue to operate normallyeven when a short circuit occurs between any one of the eighth wirings4218_1 to 4218_n and a power supply line or a signal line. Therefore, inthe shift register in FIG. 42, a range of operating conditions can beimproved. Further, in the shift register in FIG. 42, reliability can beimproved. Furthermore, in the shift register in FIG. 42, yield can beimproved. This is because the transfer signal of each flip-flop and theoutput signal of each flip-flop are divided in the shift register inFIG. 42.

Further, in a shift register to which the flip-flop of this embodimentmode is applied, advantageous effects which are similar to those of theshift registers described in Embodiment Modes 1 and 2 can be obtained.

As a display device of this embodiment mode, any of the display devicesin FIGS. 17, 19, 20, 27, and 28 can be used. Therefore, in the displaydevice of this embodiment mode, advantageous effects which are similarto those of the display devices described in Embodiment Modes 1 and 2can be obtained.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 4

In this embodiment mode, the case is described in which a P-channeltransistor is employed as a transistor included in a flip-flop of thisspecification. Further, structures and driving methods of a drivercircuit including the flip-flop and a display device including thedriver circuit are described.

In the flip-flop of this embodiment mode, the case is described in whichthe transistor included in the flip-flop in FIG. 1A is a P-channeltransistor. Therefore, in a flip-flop in FIG. 44, advantageous effectswhich are similar to those of FIG. 1A can be obtained. Note that aP-channel transistor can be employed as the transistor included in theflip-flop shown in FIG. 1B, 1C, 5A, 5B, 5C, 7A, 7B, 8A, 8B, 9A, 9B, 10A,10B, or 40. Note also that the flip-flop of this embodiment mode can befreely combined with the description of Embodiment Modes 1 to 3.

A basic structure of the flip-flop of this embodiment mode is describedwith reference to FIG. 44. A flip-flop shown in FIG. 44 includes a firsttransistor 4401, a second transistor 4402, a third transistor 4403, afourth transistor 4404, a fifth transistor 4405, a sixth transistor4406, a seventh transistor 4407, and an eighth transistor 4408. Inaddition, the first transistor 4401 to the eighth transistor 4408correspond to the first transistor 101 to the eighth transistor 108 inFIGS. 1A to 1C, respectively. Note that each of the first transistor4401 to the eighth transistor 4408 is a P-channel transistor and isturned on when the absolute value of gate-source voltage (|Vgs|) exceedsthe absolute value of the threshold voltage (|Vth|) (when Vgs becomeslower than Vth).

Note that in the flip-flop of this embodiment mode, each of the firsttransistor 4401 to the eighth transistor 4408 is a P-channel transistor.Therefore, in the flip-flop of this embodiment mode, a manufacturingprocess can be simplified. In addition, in the flip-flop of thisembodiment mode, manufacturing cost can be reduced. Further, in theflip-flop of this embodiment mode, yield can be improved.

Connection relations of the flip-flop in FIG. 44 are omitted becausethey are similar to those of FIG. 1A.

A first wiring 4421, a second wiring 4422, a third wiring 4423, a fourthwiring 4424, a fifth wiring 4425, a sixth wiring 4426, a seventh wiring4427, an eighth wiring 4428, a ninth wiring 4429, a tenth wiring 4430,an eleventh wiring 4431, a twelfth wiring 4432, a thirteenth wiring4133, a node 4441, and a node 4442 correspond to the first wiring 121,the second wiring 122, the third wiring 123, the fourth wiring 124, thefifth wiring 125, the sixth wiring 126, the seventh wiring 127, theeighth wiring 128, the ninth wiring 129, the tenth wiring 130, theeleventh wiring 131, the twelfth wiring 132, the thirteenth wiring 133,the node 141, and the node 142 in FIGS. 1A to 1C, respectively.

Next, operations of the flip-flops shown in FIG. 44 are described withreference to a timing chart in FIG. 45. Note that the timing chart inFIG. 45 is described by dividing the whole period into a set period, aselection period, a reset period, a first non-selection period, and asecond non-selection period. Note also that the set period, the resetperiod, the first non-selection period, and the second non-selectionperiod are collectively referred to as a non-selection period in somecases.

The timing chart in FIG. 45 is similar to the timing chart in FIG. 2 inwhich an H level and an L level are inverted. That is, an H level and anL level of an input signal and an output signal are just inverted in theflip-flop in FIG. 44 compared with the flip-flops in FIGS. 1A to 1C.Note that a signal 4521, a signal 4525, a signal 4528, a signal 4527, apotential 4541, a potential 4542, a signal 4522, and a signal 4523correspond to the signal 221, the signal 225, the signal 228, the signal227, the potential 241, the potential 242, the signal 222, and thesignal 223 in FIG. 2, respectively.

Note that as for power supply voltage supplied to the flip-flop in FIG.44, V1 and V2 are inverted compared with the flip-flops in FIGS. 1A to1C.

First, operations of the flip-flop in the set period shown in period Aof FIG. 45 are described. A potential of the node 4441 (the potential4541) becomes V2+|Vth4405| (Vth4405 corresponds to the threshold voltageof the fifth transistor 4405). Then, the node 4411 enters into afloating state while being kept at V2+|Vth4405|. At this time, apotential of the node 4442 becomes V1. Note that since the firsttransistor 4401 and the second transistor 4402 are on, an H-level signalis output from the third wiring 4423.

Operations of the flip-flop in the selection period shown in period B ofFIG. 45 are described. The potential of the node 4441 becomesV2−|Vth4401|−γ (Vth4401 corresponds to the threshold voltage of thefirst transistor 4401 and γ corresponds to a given positive number) by abootstrap operation. Thus, since the first transistor 4401 is turned on,an L-level signal (V2) is output from the third wiring 4423. At thistime, the potential of the node 4442 becomes V1−θ (θ corresponds to agiven positive number). In addition, θ<|Vth4406| (Vth4406 corresponds tothe threshold voltage of the sixth transistor 4406) is satisfied. Thus,the sixth transistor 4406 remains off.

Operations of the flip-flop in the reset period shown in period C ofFIG. 45 are described. Since the seventh transistor 4407 is turned on,the potential of the node 4441 becomes V1. Thus, the first transistor4401 is turned off. At this time, since the second transistor 4402 isturned on, an H-level signal is output from the third wiring 4423.

Operations of the flip-flop in the first non-selection period shown inperiod D of FIG. 45 are described. The potential of the node 4442becomes V2+|Vth4403| (Vth4403 corresponds to the threshold voltage ofthe third transistor 4403). Thus, the sixth transistor 4406 is turned onand remains at V1. At this time, the second transistor 4402 is turnedoff. Thus, since the third wiring 4423 enters into a floating state, thethird wiring 4423 remains at V1.

Operations of the flip-flop in the second non-selection period shown inperiod E of FIG. 45 are described. Since the potential of the node 4412becomes V1−θ, the sixth transistor 4406 is turned off. Thus, since thenode 4441 enters into a floating state, the node 4441 remains at V1. Atthis time, since the second transistor 4402 is turned on, an H-levelsignal (V1) is output from the third wiring 4423.

Note that in the shift register of this embodiment mode, the flip-flopof this embodiment mode can be freely combined with the shift registersdescribed in Embodiment Modes 1 to 3. For example, in the shift registerof this embodiment mode, the flip-flop of this embodiment mode can befreely combined with the shift registers in FIGS. 11, 14, 24, and 42.Note that in the shift register of this embodiment mode, an H level andan L level are inverted compared with the shift registers described inEmbodiment Modes 1 to 3.

Note that in a display device of this embodiment mode, the shiftregister of this embodiment mode can be freely combined with the displaydevices described in Embodiment Modes 1 to 3. For example, the displaydevice of this embodiment mode can be freely combined with the displaydevices in FIGS. 17, 19, 20, 27, and 28. Note that in the display deviceof this embodiment mode, an H level and an L level are inverted comparedwith the display devices described in Embodiment Modes 1 to 3.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 5

In this embodiment mode, a signal line driver circuit included in eachof the display devices shown in Embodiment Modes 1 to 4 is described.

A signal line driver circuit in FIG. 31 is described. The signal linedriver circuit shown in FIG. 31 includes a driver IC 5601, switch groups5602_1 to 5602_M, a first wiring 5611, a second wiring 5612, a thirdwiring 5613, and wirings 5621_1 to 5621_M. Each of the switch groups5602_1 to 5602_M includes a first switch 5603 a, a second switch 5603 b,and a third switch 5603 c.

The driver IC 5601 is connected to the first wiring 5611, the secondwiring 5612, the third wiring 5613, and the wirings 5621_1 to 5621_M.Each of the switch groups 5602_1 to 5602_M is connected to the firstwiring 5611, the second wiring 5612, the third wiring 5613, and thewirings 5621_1 to 5621_M corresponding to the switch groups 5602_1 to5602_M, respectively. Each of the wirings 5621_1 to 5621_M is connectedto three signal lines through the first switch 5603 a, the second switch5603 b, and the third switch 5603 c. For example, the wiring 5621_J ofthe J-th column (one of the wirings 5621_1 to 5621_M) is connected to asignal line Sj−1, a signal line Sj, and a signal line Sj+1 through thefirst switch 5603 a, the second switch 5603 b, and the third switch 5603c.

A signal is input to each of the first wiring 5611, the second wiring5612, and the third wiring 5613.

Note that the driver IC 5601 is preferably formed using a singlecrystalline substrate or a glass substrate using a polycrystallinesemiconductor. The switch groups 5602 5602_1 to 5602_M are preferablyformed over the same substrate as each pixel portion shown in EmbodimentMode 1. Therefore, the driver IC 5601 and the switch groups 5602 5602_1to 5602_M are preferably connected through an FPC or the like.

Next, operations of the signal line driver circuit shown in FIG. 31 aredescribed with reference to a timing chart in FIG. 32. The timing chartin FIG. 32 shows the case where the scan line Gi of the i-th row isselected. A selection period of the scan line Gi of the i-th row isdivided into a first sub-selection period T1, a second sub-selectionperiod T2, and a third sub-selection period T3. In addition, the signalline driver circuit in FIG. 31 operates similarly to FIG. 32 even when ascan line of another row is selected.

Note that the timing chart in FIG. 32 shows the case where the wiring5621_J in the J-th column is connected to the signal line Sj−1, thesignal line Sj, and the signal line Sj+1 through the first switch 5603a, the second switch 5603 b, and the third switch 5603 c.

The timing chart in FIG. 32 shows timing at which the scan line Gi ofthe i-th row is selected, timing 5703 a of on/off of the first switch5603 a, timing 5703 b of on/off of the second switch 5603 b, timing 5703c of on/off of the third switch 5603 c, and a signal 5721_J input to thewiring 5621_J of the J-th column.

In the first sub-selection period T1, the second sub-selection periodT2, and the third sub-selection period T3, different video signals areinput to the wirings 5621_1 to 5621_M. For example, a video signal inputto the wiring 5621_J in the first sub-selection period T1 is input tothe signal line Sj−1, a video signal input to the wiring 5621_J in thesecond sub-selection period T2 is input to the signal line Sj, and avideo signal input to the wiring 5621_J in the third sub-selectionperiod T3 is input to the signal line Sj+1. In addition, in the firstsub-selection period T1, the second sub-selection period T2, and thethird sub-selection period T3, the video signals input to the wiring5621_1 are denoted by Dataj−1, Dataj, and Dataj+1.

As shown in FIG. 32, in the first sub-selection period T1, the firstswitch 5603 a is turned on, and the second switch 5603 b and the thirdswitch 5603 c are turned off. At this time, Dataj−1 input to the wiring5621_J is input to the signal line Sj−1 through the first switch 5603 a.In the second sub-selection period T2, the second switch 5603 b isturned on, and the first switch 5603 a and the third switch 5603 c areturned off. At this time, Dataj input to the wiring 5621_J is input tothe signal line Sj through the second switch 5603 b. In the thirdsub-selection period T3, the third switch 5603 c is turned on, and thefirst switch 5603 a and the second switch 5603 b are turned off. At thistime, Dataj+1 input to the wiring 5621_J is input to the signal lineSj+1 through the third switch 5603 c.

As described above, in the signal line driver circuit in FIG. 31, bydividing one gate selection period into three, video signals can beinput to three signal lines from one wiring 5621 in one gate selectionperiod. Therefore, in the signal line driver circuit in FIG. 31, thenumber of connections of the substrate provided with the driver IC 5601and the substrate provided with the pixel portion can be approximately ⅓of the number of signal lines. The number of connections is reduced toapproximately ⅓ of the number of the signal lines, so that reliability,yield, and the like of the signal line driver circuit in FIG. 31 can beimproved.

By applying the signal line driver circuit of this embodiment mode toeach of the display devices shown in Embodiment Modes 1 to 4, the numberof connections of the substrate provided with the pixel portion and anexternal substrate can be further reduced. Therefore, reliability of thedisplay device of the present invention can be improved. In addition,yield of the display device of the present invention can be improved.

Next, the case where N-channel transistors are used for the first switch5603 a, the second switch 5603 b, and the third switch 5603 c isdescribed with reference to FIG. 33. Note that portions which aresimilar to those of FIG. 31 are denoted by common reference numerals anddetailed description of the portions which are the same and portionswhich have similar functions is omitted.

A first transistor 5903 a corresponds to the first switch 5603 a. Asecond transistor 5903 b corresponds to the second switch 5603 b. Athird transistor 5903 c corresponds to the third switch 5603 c.

For example, in the case of the switch group 5602_J, a first electrodeof the first transistor 5903 a is connected to the wiring 5621_J; asecond electrode of the first transistor 5903 a is connected to thesignal line Sj−1; and a gate electrode of the first transistor 5903 a isconnected to the first wiring 5611. A first electrode of the secondtransistor 5903 b is connected to the wiring 5621_J; a second electrodeof the second transistor 5903 b is connected to the signal line Sj; anda gate electrode of the second transistor 5903 b is connected to thesecond wiring 5612. A first electrode of the third transistor 5903 c isconnected to the wiring 5621_J; a second electrode of the thirdtransistor 5903 c is connected to the signal line Sj+1; and a gateelectrode of the third transistor 5903 c is connected to the thirdwiring 5613.

Note that each of the first transistor 5903 a, the second transistor5903 b, and the third transistor 5903 c functions as a switchingtransistor. Further, each of the first transistor 5903 a, the secondtransistor 5903 b, and the third transistor 5903 c is turned on when asignal input to each gate electrode is at an H level, and is turned offwhen a signal input to each gate electrode is at an L level.

When N-channel transistors are used for the first switch 5603 a, thesecond switch 5603 b, and the third switch 5603 c, amorphous silicon canbe used for a semiconductor layer of each transistor. Therefore, amanufacturing process can be simplified, and thus manufacturing cost canbe reduced and yield can be improved. Further, a semiconductor devicesuch as a large display panel can be formed. Even when polysilicon orsingle crystalline silicon is used for the semiconductor layer of eachtransistor, the manufacturing process can be simplified.

In the signal line driver circuit in FIG. 33, N-channel transistors areused for the first transistor 5903 a, the second transistor 5903 b, andthe third transistor 5903 c; however, P-channel transistors may be usedfor the first transistor 5903 a, the second transistor 5903 b, and thethird transistor 5903 c. In this case, each transistor is turned on whena signal input to the gate electrode is at an L level, and is turned offwhen a signal input to the gate electrode is at an H level.

Note that arrangement, the number, a driving method, and the like of theswitches are not limited as long as one gate selection period is dividedinto a plurality of sub-selection periods and video signals are input toa plurality of signal lines from one wiring in each of the plurality ofsub-selection periods as shown in FIG. 31.

For example, when video signals are input to three or more signal linesfrom one wiring in each of three or more sub-selection periods, it isonly necessary to add a switch and a wiring for controlling the switch.Note that when one selection period is divided into four or moresub-selection periods, one sub-selection period becomes short.Therefore, one selection period is preferably divided into two or threesub-selection periods.

As another example, one selection period may be divided into a prechargeperiod Tp, the first sub-selection period T1, the second sub-selectionperiod T2, and the third sub-selection period T3 as shown in a timingchart in FIG. 34. The timing chart in FIG. 34 shows timing at which thescan line Gi of the i-th row is selected, timing 5803 a of on/off of thefirst switch 5603 a, timing 5803 b of on/off of the second switch 5603b, timing 5803 c of on/off of the third switch 5603 c, and a signal5821_J input to the wiring 5621_J of the J-th column. As shown in FIG.34, the first switch 5603 a, the second switch 5603 b, and the thirdswitch 5603 c are tuned on in the precharge period Tp. At this time,precharge voltage Vp input to the wiring 5621_J is input to each of thesignal line Sj−1, the signal line Sj, and the signal line Sj+1 throughthe first switch 5603 a, the second switch 5603 b, and the third switch5603 c. In the first sub-selection period T1, the first switch 5603 a isturned on, and the second switch 5603 b and the third switch 5603 c areturned off. At this time, Dataj−1 input to the wiring 5621_J is input tothe signal line Sj−1 through the first switch 5603 a. In the secondsub-selection period T2, the second switch 5603 b is turned on, and thefirst switch 5603 a and the third switch 5603 c are turned off. At thistime, Dataj input to the wiring 5621_J is input to the signal line Sjthrough the second switch 5603 b. In the third sub-selection period T3,the third switch 5603 c is turned on, and the first switch 5603 a andthe second switch 5603 b are turned off. At this time, Dataj+1 input tothe wiring 56211 is input to the signal line Sj+1 through the thirdswitch 5603 c.

As described above, in the signal line driver circuit in FIG. 31 towhich the timing chart in FIG. 34 is applied, the video signal can bewritten to the pixel at high speed because the signal line can beprecharged by providing a precharge selection period before asub-selection period. Note that portions which are similar to those ofFIG. 32 are denoted by common reference numerals and detaileddescription of the portions which are the same and portions which havesimilar functions is omitted.

As shown in FIG. 31, one gate selection period can be divided into aplurality of sub-selection periods and video signals can be input to aplurality of signal lines from one wiring in each of the plurality ofsub-selection periods also in FIG. 35. Note that FIG. 35 shows only aswitch group 6022_J of the J-th column in a signal line driver circuit.The switch group 6022_J includes a first transistor 6001, a secondtransistor 6002, a third transistor 6003, a fourth transistor 6004, afifth transistor 6005, and a sixth transistor 6006. The first transistor6001, the second transistor 6002, the third transistor 6003, the fourthtransistor 6004, the fifth transistor 6005, and the sixth transistor6006 are N-channel transistors. The switch group 6022_J is connected toa first wiring 6011, a second wiring 6012, a third wiring 6013, a fourthwiring 6014, a fifth wiring 6015, a sixth wiring 6016, the wiring5621_J, the signal line Sj−1, the signal line Sj, and the signal lineSj+1.

A first electrode of the first transistor 6001 is connected to thewiring 5621_J; a second electrode of the first transistor 6001 isconnected to the signal line Sj−1; and a gate electrode of the firsttransistor 6001 is connected to the first wiring 6011. A first electrodeof the second transistor 6002 is connected to the wiring 5621_J; asecond electrode of the second transistor 6002 is connected to thesignal line Sj−1; and a gate electrode of the second transistor 6002 isconnected to the second wiring 6012. A first electrode of the thirdtransistor 6003 is connected to the wiring 5621_J; a second electrode ofthe third transistor 6003 is connected to the signal line Sj; and a gateelectrode of the third transistor 6003 is connected to the third wiring6013. A first electrode of the fourth transistor 6004 is connected tothe wiring 5621_J; a second electrode of the fourth transistor 6004 isconnected to the signal line Sj; and a gate electrode of the fourthtransistor 6004 is connected to the fourth wiring 6014. A firstelectrode of the fifth transistor 6005 is connected to the wiring5621_J; a second electrode of the fifth transistor 6005 is connected tothe signal line Sj+1; and a gate electrode of the fifth transistor 6005is connected to the fifth wiring 6015. A first electrode of the sixthtransistor 6006 is connected to the wiring 5621_J; a second electrode ofthe sixth transistor 6006 is connected to the signal line Sj+1; and agate electrode of the sixth transistor 6006 is connected to the sixthwiring 6016.

Note that each of the first transistor 6001, the second transistor 6002,the third transistor 6003, the fourth transistor 6004, the fifthtransistor 6005, and the sixth transistor 6006 functions as a switchingtransistor. Further, each of first transistor 6001, the secondtransistor 6002, the third transistor 6003, the fourth transistor 6004,the fifth transistor 6005, and the sixth transistor 6006 is turned onwhen a signal input to each gate electrode is at an H level, and isturned off when a signal input to each gate electrode is at an L level.

Note that the first wiring 6011 and the second wiring 6012 correspond toa first wiring 5913 in FIG. 33. The third wiring 6013 and the fourthwiring 6014 correspond to a second wiring 5912 in FIG. 33. The fifthwiring 6015 and the sixth wiring 6016 correspond to a third wiring 5911in FIG. 33. The first transistor 6001 and the second transistor 6002correspond to the first transistor 5903 a in FIG. 33. The thirdtransistor 6003 and the fourth transistor 6004 correspond to the secondtransistor 5903 b in FIG. 33. The fifth transistor 6005 and the sixthtransistor 6006 correspond to the third transistor 5903 c in FIG. 33.

In FIG. 35, in the first sub-selection period T1 shown in FIG. 32, oneof the first transistor 6001 and the second transistor 6002 is turnedon. In the second sub-selection period T2, one of the third transistor6003 and the fourth transistor 6004 is turned on. In the thirdsub-selection period T3, one of the fifth transistor 6005 and the sixthtransistor 6006 is turned on. Further, in the precharge period Tp shownin FIG. 34, either the first transistor 6001, the third transistor 6003,and the fifth transistor 6005; or the second transistor 6002, the fourthtransistor 6004, and the sixth transistor 6006 are turned on.

Therefore, in FIG. 35, since on time of each transistor can beshortened, deterioration in characteristics of the transistor can besuppressed. This is because in the first sub-selection period T1 shownin FIG. 32, for example, the video signal can be input to the signalline Sj−1 when one of the first transistor 6001 and the secondtransistor 6002 is turned on. Note that in the first sub-selectionperiod T1 shown in FIG. 32, for example, when both the first transistor6001 and the second transistor 6002 are turned on at the same time, thevideo signal can be input to the signal line Sj−1 at high speed.

Note that although two transistors are connected in parallel between thewiring 5621 and the signal line in FIG. 35, the present invention is notlimited to this, and three or more transistors may be connected inparallel between the wiring 5621 and the signal line. Thus,deterioration in characteristics of each transistor can be furthersuppressed.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 6

In this embodiment mode, a structure for preventing a defect due toelectrostatic discharge in the display device shown in Embodiment Modes1 to 4 is described.

Note that electrostatic discharge corresponds to instant dischargethrough an input/output terminal of a semiconductor device when positiveor negative charges stored in the human body or the object touch thesemiconductor device, and damage caused by supplying large currentflowing within the semiconductor device.

FIG. 36A shows a structure for preventing electrostatic discharge causedin a scan line by a protective diode. FIG. 36A shows a structure wherethe protective diode is provided between a wiring 6111 and the scanline. Although not shown, a plurality of pixels are connected to thescan line Gi of the i-th row. Note that a transistor 6101 is used as theprotective diode. Although the transistor 6101 is an N-channeltransistor, a P-channel transistor may be used, and polarity of thetransistor 6101 may be the same as that of a transistor included in ascan line driver circuit or a pixel.

Note that although one protective diode is arranged here, a plurality ofprotective diodes may be arranged in series, in parallel, or inserial-parallel.

A first electrode of the transistor 6101 is connected to the scan lineGi of the i-th row; a second electrode of the transistor 6101 isconnected to the wiring 6111; and a gate electrode of the transistor6101 is connected to the scan line Gi of the i-th row.

Operations of FIG. 36A are described. A certain potential is input tothe wiring 6111, which is lower than an L level of a signal input to thescan line Gi of the i-th row. When positive or negative charge is notdischarged to the scan line Gi of the i-th row, a potential of the scanline Gi of the i-th row is at an H level or an L level, so that thetransistor 6101 is off. On the other hand, when negative charge isdischarged to the scan line Gi of the i-th row, the potential of thescan line Gi of the i-th row lowers instantaneously. At this time, whenthe potential of the scan line Gi of the i-th row is lower than a valueobtained by subtracting the threshold voltage of the transistor 6101from a potential of the wiring 6111, the transistor 6101 is turned onand current flows to the wiring 6111 through the transistor 6101.Therefore, the structure shown in FIG. 36A can prevent large currentfrom flowing to the pixel, so that electrostatic discharge of the pixelcan be prevented.

FIG. 36B shows a structure for preventing electrostatic discharge whenpositive charge is discharged to the scan line Gi of the i-th row. Atransistor 6102 functioning as a protective diode is provided betweenthe scan line and a wiring 6112. Note that although one protective diodeis arranged here, a plurality of protective diodes may be arranged inseries, in parallel, or in serial-parallel. Although the transistor 6102is an N-channel transistor, a P-channel transistor may be used, andpolarity of the transistor 6102 may be the same as that of thetransistor included in the scan line driver circuit or the pixel. Afirst electrode of the transistor 6102 is connected to the scan line Giof the i-th row; a second electrode of the transistor 6102 is connectedto the wiring 6112; and a gate electrode of the transistor 6102 isconnected to the wiring 6112. Note that a potential higher than an Hlevel of the signal input to the scan line Gi of the i-th row is inputto the wiring 6112. Therefore, when charge is not discharged to the scanline Gi of the i-th row, the transistor 6102 is off. On the other hand,when positive charge is discharged to the scan line Gi of the i-th row,the potential of the scan line Gi of the i-th row rises instantaneously.At this time, when the potential of the scan line Gi of the i-th row ishigher than the sum of a potential of the wiring 6112 and the thresholdvoltage of the transistor 6102, the transistor 6102 is turned on andcurrent flows to the wiring 6112 through the transistor 6102. Therefore,the structure shown in FIG. 36B can prevent large current from flowingto the pixel, so that electrostatic discharge of the pixel can beprevented.

As shown in FIG. 36C, with a structure which combines FIGS. 36A and 36B,electrostatic discharge of the pixel can be prevented when positive ornegative charge is discharged to the scan line Gi of the i-th row. Notethat portions which are similar to those of FIGS. 36A and 36B aredenoted by common reference numerals, and detailed description of theportions which are the same and portions which have similar functions isomitted.

FIG. 37A shows a structure where a transistor 6201 functioning as aprotective diode is connected between a scan line and a storagecapacitor line. Note that although one protective diode is arrangedhere, a plurality of protective diodes may be arranged in series, inparallel, or in serial-parallel. Although the transistor 6201 is anN-channel transistor, a P-channel transistor may be used, and polarityof the transistor 6201 may be the same as that of the transistorincluded in the scan line driver circuit or the pixel. A wiring 6211functions as a storage capacitor line. A first electrode of thetransistor 6201 is connected to the scan line Gi of the i-th row; asecond electrode of the transistor 6201 is connected to the wiring 6211;and a gate electrode of the transistor 6201 is connected to the scanline Gi of the i-th row. Note that a potential lower than an L level ofthe signal input to the scan line Gi of the i-th row is input to thewiring 6211. Therefore, when charge is not discharged to the scan lineGi of the i-th row, the transistor 6210 is off. On the other hand, whennegative charge is discharged to the scan line Gi of the i-th row, thepotential of the scan line Gi of the i-th row lowers instantaneously. Atthis time, when the potential of the scan line Gi of the i-th row islower than a value obtained by subtracting the threshold voltage of thetransistor 6201 from a potential of the wiring 6211, the transistor 6201is turned on and current flows to the wiring 6211 through the transistor6201. Therefore, the structure shown in FIG. 37A can prevent largecurrent from flowing to the pixel, so that electrostatic discharge ofthe pixel can be prevented. Further, since the storage capacitor line isutilized as a wiring for discharging charge in the structure shown inFIG. 37A, it is not necessary to add a wiring.

FIG. 37B shows a structure for preventing electrostatic discharge whenpositive charge is discharged to the scan line Gi of the i-th row. Here,a potential higher than an H level of the signal input to the scan lineGi of the i-th row is input to the wiring 6211. Therefore, when chargeis not discharged to the scan line Gi of the i-th row, the transistor6202 is off. On the other hand, when positive charge is discharged tothe scan line Gi of the i-th row, the potential of the scan line Gi ofthe i-th row rises instantaneously. At this time, when the potential ofthe scan line Gi of the i-th row is higher than the sum of a potentialof the wiring 6211 and the threshold voltage of the transistor 6202, thetransistor 6202 is turned on and current flows to the wiring 6211through the transistor 6202. Therefore, the structure shown in FIG. 37Bcan prevent large current from flowing to the pixel, so thatelectrostatic discharge of the pixel can be prevented. Further, sincethe storage capacitor line is utilized as a wiring for dischargingcharge in the structure shown in FIG. 37B, it is not necessary to add awiring. Note that portions which are similar to those of FIG. 37A aredenoted by common reference numerals, and detailed description of theportions which are the same and portions which have similar functions isomitted.

Next, FIG. 38A shows a structure for preventing electrostatic dischargecaused in a signal line by a protective diode. FIG. 38A shows astructure where the protective diode is provided between a wiring 6411and the signal line. Although not shown, a plurality of pixels areconnected to the signal line Sj of the j-th column. A transistor 6401 isused as the protective diode. Note that although the transistor 6401 isan N-channel transistor, a P-channel transistor may be used, andpolarity of the transistor 6401 may be the same as that of a transistorincluded in a signal line driver circuit or the pixel.

Note that although one protective diode is arranged here, a plurality ofprotective diodes may be arranged in series, in parallel, or inserial-parallel.

A first electrode of the transistor 6401 is connected to the signal lineSj of the j-th column; a second electrode of the transistor 6401 isconnected to the wiring 6411; and a gate electrode of the transistor6401 is connected to the signal line Sj of the j-th column.

Operations of FIG. 38A are described. A certain potential is input tothe wiring 6411, which is lower than the smallest value of a videosignal input to the signal line Sj of the j-th column. When positive ornegative charge is not discharged to the signal line Sj of the j-thcolumn, a potential of the signal line Sj of the j-th column is the sameas the video signal, so that the transistor 6401 is off. On the otherhand, when negative charge is discharged to the signal line Sj of thej-th column, the potential of the signal line Sj of the j-th columnlowers instantaneously. At this time, when the potential of the signalline Sj of the j-th column is lower than a value obtained by subtractingthe threshold voltage of the transistor 6401 from a potential of thewiring 6411, the transistor 6401 is turned on and current flows to thewiring 6411 through the transistor 6401. Therefore, the structure shownin FIG. 38A can prevent large current from flowing to the pixel, so thatelectrostatic discharge of the pixel can be prevented.

FIG. 38B shows a structure for preventing electrostatic discharge whenpositive charge is discharged to the signal line Sj of the j-th column.A transistor 6402 functioning as a protective diode is provided betweenthe signal line and a wiring 6412. Note that although one protectivediode is arranged here, a plurality of protective diodes may be arrangedin series, in parallel, or in serial-parallel. Although the transistor6402 is an N-channel transistor, a P-channel transistor may be used, andpolarity of the transistor 6402 may be the same as that of thetransistor included in the signal line driver circuit or the pixel. Afirst electrode of the transistor 6402 is connected to the signal lineSj of the j-th column; a second electrode of the transistor 6402 isconnected to the wiring 6412; and a gate electrode of the transistor6402 is connected to the wiring 6412. Note that a potential higher thanthe largest value of a video signal input to the signal line Sj of thej-th column is input to the wiring 6412. Therefore, when charge is notdischarged to the signal line Sj of the j-th column, the transistor 6402is off. On the other hand, when positive charge is discharged to thesignal line Sj of the j-th column, the potential of the signal line Sjof the j-th column rises instantaneously. At this time, when thepotential of the signal line Sj of the j-th column is higher than thesum of a potential of the wiring 6412 and the threshold voltage of thetransistor 6402, the transistor 6402 is turned on and current flows tothe wiring 6412 through the transistor 6402. Therefore, the structureshown in FIG. 38B can prevent large current from flowing to the pixel,so that electrostatic discharge of the pixel can be prevented.

As shown in FIG. 38C, with a structure which combines FIGS. 38A and 38B,electrostatic discharge of the pixel can be prevented when positive ornegative charge is discharged to the signal line Sj of the j-th column.Note that portions which are similar to those of FIGS. 38A and 38B aredenoted by common reference numerals, and detailed description of theportions which are the same and portions which have similar functions isomitted.

In this embodiment mode, the structures for preventing electrostaticdischarge of the pixel connected to the scan line and the signal lineare described. However, the structures of this embodiment mode are notonly used for preventing electrostatic discharge of the pixel connectedto the scan line and the signal line. For example, when this embodimentmode is used for the wiring to which a signal or a potential is input,connected to the scan line driver circuit and the signal line drivercircuit shown in Embodiment Modes 1 to 4, electrostatic discharge of thescan line driver circuit and the signal line driver circuit can beprevented.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 7

In this embodiment mode, another structure of a display device which canbe applied to each of the display devices shown in Embodiment Modes 1 to4 is described.

FIG. 39A shows a structure where a diode-connected transistor isprovided between a scan line and another scan line. FIG. 39A shows astructure where a diode-connected transistor 6301 a is provided betweenthe scan line Gi−1 of the (i−1)th row and the scan line Gi of the i-throw, and a diode-connected transistor 6301 b is provided between thescan line Gi of the i-th row and the scan line Gi+1 of the (i+1)th row.Note that although the transistors 6301 a and 6301 b are N-channeltransistors, P-channel transistors may be used, and polarity of thetransistors 6301 a and 6301 b may be the same as that of a transistorincluded in a scan line driver circuit or a pixel.

Note that in FIG. 39A, the scan line Gi−1 of the (i−1)th row, the scanline Gi of the i-th row, and the scan line Gi+1 of the (i+1)th row aretypically shown, and a diode-connected transistor is similarly providedbetween other scan lines.

A first electrode of the transistor 6301 a is connected to the scan lineGi of the i-th row; a second electrode of the transistor 6301 a isconnected to the scan line Gi−1 of the (i−1)th row; and a gate electrodeof the transistor 6301 a is connected to the scan line Gi−1 of the(i−1)th row. A first electrode of the transistor 6301 b is connected tothe scan line Gi+1 of (i+1)th row; a second electrode of the transistor6301 b is connected to the scan line Gi of the i-th row; and a gateelectrode of the transistor 6301 b is connected to the scan line Gi ofthe i-th row.

Operations of FIG. 39A are described. In each of the scan line drivercircuits shown in Embodiment Modes 1 to 4, the scan line Gi−1 of the(i−1)th row, the scan line Gi of the i-th row, and the scan line Gi+1 ofthe (i+1)th row remain at an L level in the non-selection period.Therefore, the transistors 6301 a and 6301 b are off. However, when thepotential of the scan line Gi of the i-th row is raised due to noise orthe like, for example, a pixel is selected by the scan line Gi of thei-th row and a wrong video signal is written to the pixel. Accordingly,by providing the diode-connected transistor between the scan lines asshown in FIG. 39A, writing of a wrong video signal to the pixel can beprevented. This is because when the potential of the scan line Gi of thei-th row rises to equal to or higher than the sum of a potential of thescan line Gi−1 of the (i−1)th row and the threshold voltage of thetransistor 6301 a, the transistor 6301 a is turned on and the potentialof the scan line Gi of i-th row lowers. Therefore, the pixel is notselected by the scan line Gi of i-th row.

The structure of FIG. 39A is particularly advantageous when a scan linedriver circuit and a pixel portion are formed over the same substrate.This is because in the scan line driver circuit including only N-channeltransistors or only P-channel transistors, a scan line is sometimesenters into a floating state and noise easily occurs in the scan line.

FIG. 39B shows a structure where a direction of a diode-connectedtransistor provided between the scan lines is reversed to that in FIG.39A. Note that although transistors 6302 a and 6302 b are N-channeltransistors, P-channel transistors may be used, and polarity of thetransistors 6302 a and 6302 b may be the same as that of the transistorincluded in the scan line driver circuit or the pixel. In FIG. 39B, afirst electrode of the transistor 6302 a is connected to the scan lineGi of the i-th row; a second electrode of the transistor 6302 a isconnected to the scan line Gi−1 of the (i−1)th row; and a gate electrodeof the transistor 6302 a is connected to the scan line Gi of the i-throw. A first electrode of the transistor 6302 b is connected to the scanline Gi+1 of (i+1)th row; a second electrode of the transistor 6302 b isconnected to the scan line Gi of the i-th row; and a gate electrode ofthe transistor 6302 b is connected to the scan line Gi+1 of (i+1)th row.In FIG. 39B, similarly to FIG. 38A, when the potential of the scan lineGi of the i-th row rises to equal to or higher than the sum of thepotential of the scan line Gi+1 of (i+1)th row and the threshold voltageof the transistor 6302 b, the transistor 6302 b is turned on and thepotential of the scan line Gi of the i-th row lowers. Therefore, thepixel is not selected by the scan line Gi of the i-th row, and writingof a wrong video signal to the pixel can be prevented.

As shown in FIG. 39C, with a structure which combines FIGS. 39A and 39B,even when the potential of the scan line Gi of the i-th row rises, thetransistors 6301 a and 6301 b are tuned on and the potential of the scanline Gi of the i-th row lowers. Note that in FIG. 39C, since currentflows through two transistors, larger noise can be removed. Note thatportions which are similar to those of FIGS. 39A and 39B are denoted bycommon reference numerals, and detailed description of the portionswhich are the same and portions which have similar functions is omitted.

Note that as shown in FIGS. 37A and 37B, when a diode-connectedtransistor is provided between the scan line and the storage capacitorline, advantageous effects which are similar to those of FIGS. 39A to39C can be obtained.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 8

In this embodiment mode, a structure and a manufacturing method of atransistor are described.

FIGS. 46A to 46G are cross-sectional views showing examples of astructure and a manufacturing method of a transistor. FIG. 46A is across-sectional view showing a structural example of the transistor.FIGS. 46B to 46G are cross-sectional views showing an example of amanufacturing method of the transistor.

The structure and the manufacturing method of the transistor are notlimited to those shown in FIGS. 46A to 46G, and various structures andmanufacturing methods can be employed.

A structural example of a transistor is described with reference to FIG.46A. FIG. 46A is a cross-sectional view of a plurality of transistorshaving different structures. In FIG. 46A, although the plurality of thetransistors having different structures are arranged, this arrangementis made for describing the structures of the transistors, and it is notnecessary to arrange the transistors actually as shown in FIG. 46A, andthe transistors can be arranged as necessary.

Then, layers which form a transistor are each described.

A substrate 110111 can be a glass substrate such as a bariumborosilicate glass, an alumino borosilicate glass, a quartz substrate, aceramic substrate, or a metal substrate including stainless steel, forexample. Besides these, a substrate formed of a synthetic resin havingflexibility such as acrylic or plastic represented by polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), andpolyethersulfone (PES) can be also used. By using such a flexiblesubstrate, a semiconductor device which can be bent can be formed. Sincea flexible substrate has no restrictions on an area and a shape of asubstrate to be used, a rectangular substrate with a side of one meteror more is used as the substrate 110111, for example, so thatproductivity can be significantly improved. Such a merit is greatlyadvantageous over the case of using a circular silicon substrate.

An insulating film 110112 functions as a base film. The insulating film110112 is provided to prevent alkali metal such as Na or alkaline earthmetal from the substrate 110111 from adversely affecting characteristicsof a semiconductor element. The insulating film 110112 can have asingle-layer structure or a stacked-layer structure of an insulatingfilm including oxygen or nitrogen, such as silicon oxide (SiO_(x)),silicon nitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y), x>y), orsilicon nitride oxide (SiN_(x)O_(y), x>y). For example, when theinsulating film 110112 is provided to have a two-layer structure, it ispreferable that a silicon nitride oxide film be used as a firstinsulating film and a silicon oxynitride film be used as a secondinsulating film. When the insulating film 110112 is provided to have athree-layer structure, it is preferable that a silicon oxynitride filmbe used as a first insulating film, a silicon nitride oxide film be usedas a second insulating film, and a silicon oxynitride film be used as athird insulating film.

Semiconductor layers 110113, 110114, and 110115 can be formed using anamorphous semiconductor, a microcrystalline semiconductor or asemi-amorphous semiconductor (SAS). Alternatively, a polycrystallinesemiconductor film may be used. SAS is a semiconductor having anintermediate structure between amorphous and crystalline (includingsingle crystalline and polycrystalline) structures and having a thirdstate which is stable in free energy. Moreover, SAS includes acrystalline region with a short range order and lattice distortion. Acrystalline region of 0.5 to 20 nm can be observed at least in part ofan SAS film. When silicon is contained as a main component, Ramanspectrum shifts to a wave number side lower than 520 cm⁻¹. Thediffraction peaks of (111) and (220) which are thought to be derivedfrom a silicon crystalline lattice are observed by X-ray diffraction.SAS contains hydrogen or halogen of at least 1 atomic % or more toterminate dangling bonds. SAS is formed by glow discharge decomposition(plasma CVD) of a material gas. As the material gas, Si₂H₆, SiH₂Cl₂,SiHCl₃, SiCl₄, SiF₄, or the like can be used in addition to SiH₄.Further, GeF₄ may be mixed. Alternatively, the material gas may bediluted with H₂, or H₂ and one or more kinds of rare gas elementsselected from He, Ar, Kr, and Ne. A dilution ratio may be in the rangeof 2 to 1000 times, pressure may be in the range of approximately 0.1 to133 Pa, a power supply frequency may be 1 to 120 MHz and preferably 13to 60 MHz, and a substrate heating temperature may be 300° C. or tower.A concentration of impurities in atmospheric components such as oxygen,nitrogen, and carbon is preferably 1×10²⁰ cm⁻¹ or less as impurityelements in the film. In particular, an oxygen concentration is5×10¹⁹/cm³ or less, and preferably 1×10¹⁹/cm³ or less. Here, anamorphous silicon film is formed using a material including silicon (Si)as its main component (e.g., Si_(x)Ge_(1−x)) by a known method (e.g., asputtering method, an LPCVD method, or a plasma CVD method). Then, theamorphous silicon film is crystallized by a known crystallization methodsuch as a laser crystallization method, a thermal crystallization methodusing RTA or an annealing furnace, or a thermal crystallization methodusing a metal element which promotes crystallization.

An insulating film 110116 can have a single-layer structure or astacked-layer structure of an insulating film(s) including oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y), x>y), or silicon nitride oxide(SiN_(x)O_(y), x>y).

A gate electrode 110117 can have a single-layer structure of aconductive film or a stacked-layer structure of two or three conductivefilms. As a material for the gate electrode 110117, a conductive filmcan be used. For example, a film of an element such as tantalum (Ta),titanium (Ti), molybdenum (Mo), tungsten (W), chromium (Cr), or silicon(Si); a nitride film including the element (typically, a tantalumnitride film, a tungsten nitride film, or a titanium nitride film); analloy film in which the elements are combined (typically, a Mo—W alloyor a Mo—Ta alloy); a silicide film including the element (typically, atungsten silicide film or a titanium silicide film); and the like can beused. Note that the above-described film of such an element, nitridefilm, alloy film, silicide film, and the like can have a single-layerstructure or a stacked-layer structure.

An insulating film 110118 can have a single-layer structure or astacked-layer structure of an insulating film including oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiN_(x)),silicon oxynitride (SiO_(x)N_(y), x>y), or silicon nitride oxide(SiN_(x)O_(y), x>y); or a film including carbon, such as a DLC (DiamondLike Carbon), by a sputtering method or a plasma CVD method.

An insulating film 110119 can have a single-layer structure or astacked-layer structure of a siloxane resin; an insulating filmincluding oxygen or nitrogen, such as silicon oxide (SiOx), siliconnitride (SiN_(x)), silicon oxynitride (SiO_(x)N_(y), x>y), or siliconnitride oxide (SiN_(x)O_(y), x>y); or a film including carbon, such as aDLC (Diamond-Like Carbon); an organic material such as epoxy, polyimide,polyamide, polyvinyl phenol, benzocyclobutene, or acrylic. Note that thesiloxane resin corresponds to a resin having Si—O—Si bonds. Siloxaneincludes a skeleton structure of a bond of silicon (Si) and oxygen (O).As a substituent, an organic group including at least hydrogen (e.g., asan alkyl group or aromatic hydrocarbon) is used. Alternatively, a fluorogroup, or a fluoro group and an organic group including at leasthydrogen can be used as a substituent. Note that the insulating film110119 can be provided to cover the gate electrode 110117 directlywithout provision of the insulating film 110118.

As a conductive film 110123, a film of an element such as Al, Ni, C, W,Mo, Ti, Pt, Cu, Ta, Au, or Mn, a nitride film including the element, analloy film in which the elements are combined, a silicide film includingthe element, or the like can be used. For example, as an alloy includingsome of such elements, an Al alloy including C and Ti, an Al alloyincluding Ni, an Al alloy including C and Ni, an Al alloy including Cand Mn, or the like can be used. In the case of a stacked-layerstructure, for example, a structure can be such that Al is interposedbetween Mo, Ti, or the like, so that resistance of Al to heat andchemical reaction can be improved.

Next, characteristics of each structure is described with reference tothe cross-sectional view of the plurality of transistors each having adifferent structure in FIG. 46A.

A transistor 110101 is a single drain transistor. Since it can be formedby a simple method, it is advantageous in low manufacturing cost andhigh yield. Note the taper angel is equal to or larger than 45° tosmaller than 95°, more preferably, equal to or larger than 60° tosmaller than 95°. Alternatively, the taper angle may be smaller than45°. Here, the semiconductor layers 110113 and 110115 each havedifferent concentration of impurities, and the semiconductor layer110113 is used as a channel region and the semiconductor layers 110115are used as a source region and a drain region. By controlling theamount of impurities in this manner, resistivity of the semiconductorlayer can be controlled. Further, an electrical connection state betweenthe semiconductor layer and the conductive film 110123 can be closer toohmic contact. Note that as a method of separately forming thesemiconductor layers each including different amount of impurities, amethod where impurities are added to the semiconductor layer using thegate electrode 110117 as a mask can be used.

A transistor 110102 denotes a transistor in which the gate electrode110117 has a certain tapered angle or more. Since it can be formed by asimple method, it is advantageous in low manufacturing cost and highyield. Here, the semiconductor layers 110111, 110114, and 10115 eachhave different concentration of impurities. The semiconductor layer110113 is used as a channel region, the semiconductor layers 110114 aslightly doped drain (LDD) regions, and the semiconductor layers 110115as a source region and a drain region. By controlling the amount ofimpurities in this manner, resistivity of the semiconductor layer can becontrolled. Further, an electrical connection state between thesemiconductor layer and the conductive film 110123 can be closer toohmic contact. Moreover, since the transistor includes the LDD region,high electric field is hardly applied to the transistor, so thatdeterioration of the element due to hot carriers can be suppressed. Notethat as a method of separately forming the semiconductor layers eachincluding different amount of impurities, a method where impurities areadded to the semiconductor layer using the gate electrode 110117 as amask can be used. In the transistor 110102, since the gate electrode110117 has a certain tapered angle or more, gradient of theconcentration of impurities added to the semiconductor layer through thegate electrode 110117 can be provided, and the LDD region can be easilyformed. Note the taper angel is equal to or larger than 45° to smallerthan 95°, more preferably, equal to or larger than 60° to smaller than95°. Alternatively, the taper angle may be smaller than 45°.

A transistor 110103 denotes a transistor in which the gate electrode110117 includes at least two layers and a lower gate electrode is longerthan an upper gate electrode. In this specification, the shape of theupper gate electrode and the lower gate electrode is referred to as ahat shape. When the gate electrode 110117 has such a hat shape, an LDDregion can be formed without addition of a photomask. Note that astructure where the LDD region overlaps with the gate electrode 110117,like the transistor 110103, is particularly called a GOLD (GateOverlapped LDD) structure. As a method of forming the gate electrode110117 with such a hat shape, the following method may be used.

First, when the gate electrode 110117 is patterned, the lower and uppergate electrodes are etched by dry etching so that side surfaces thereofare inclined (tapered). Then, the inclination of the upper gateelectrode is processed to be almost perpendicular by anisotropicetching. Thus, the gate electrode is formed such that the cross sectionis hat-shaped. Then, doping of impurity elements is conducted twice, sothat the semiconductor layer 110113 used as a channel region, thesemiconductor layers 110114 used as LDD regions, and the semiconductorlayers 110115 used as a source electrode and a drain electrode areformed.

Note that a portion of the LDD region, which overlaps with the gateelectrode 110117, is referred to as an Lov region, and a portion of theLDD region, which does not overlap with the gate electrode 110117, isreferred to as an Loff region. The Loff region is highly effective insuppressing an off-current value, whereas it is not very effective inpreventing deterioration in an on-current value due to hot carriers byrelieving an electric field in the vicinity of the drain. On the otherhand, the Lov region is highly effective in preventing deterioration inthe on-current value by relieving the electric field in the vicinity ofthe drain, whereas it is not very effective in suppressing theoff-current value. Thus, it is preferable to form a transistor having astructure corresponding to characteristics required for each of thevarious circuits. For example, when the semiconductor device is used fora display device, a transistor having an Loff region is preferably usedas a pixel transistor in order to suppress the off-current value. On theother hand, as a transistor in a peripheral circuit, a transistor havingan Lov region is preferably used in order to prevent deterioration inthe on-current value by relieving the electric field in the vicinity ofthe drain.

A transistor 110104 denotes a transistor including a sidewall 110121 incontact with a side surface of the gate electrode 110117. When thetransistor includes the sidewall 110121, a region overlapping with thesidewall 110121 can be formed as an LDD region.

A transistor 110105 denotes a transistor in which an LDD (Loft) regionis formed by doping the semiconductor layer with an impurity element,using a mask 110122. Thus, the LDD region can surely be formed, and anoff-current value of the transistor can be reduced.

A transistor 110106 denotes a transistor in which an LDD (Lov) region isformed by doping in the semiconductor layer with use of a mask. Thus,the LDD region can surely be formed, and deterioration in an on-currentvalue can be prevented by relieving the electric field in the vicinityof the drain of the transistor.

Next, an example of a manufacturing method of a transistor is describedwith reference to FIGS. 46B to 46G.

Note that a structure and a manufacturing method of a transistor are notlimited to those in FIGS. 46A to 46G, and various structures andmanufacturing methods can be used.

In this embodiment mode, a surface of the substrate 110111, theinsulating film 110112, the semiconductor layer 110113, thesemiconductor layer 110114, the semiconductor layer 110115, theinsulating film 110116, the insulating film 110118, or the insulatingfilm 110119 is oxidized or nitrided by plasma treatment, so that thesemiconductor layer or the insulating film can be oxidized or nitrided.By oxidizing or nitriding the semiconductor layer or the insulating filmby plasma treatment in such a manner, a surface of the semiconductorlayer or the insulating film is modified, and the insulating film can beformed to be denser than an insulating film formed by a CVD method or asputtering method; thus, a defect such as a pinhole can be suppressed,and characteristics and the like of the semiconductor device can beimproved.

Note that silicon oxide (SiO_(x)) or silicon nitride (SiN_(x)) can beused for the sidewall 110121. As a method of forming the sidewall 110121on the side surface of the gate electrode 110117, a method in which thegate electrode 110117 is formed, then, a silicon oxide (SiO_(x)) film ora silicon nitride (SiN_(x)) film is formed, and then, the silicon oxide(SiO_(x)) film or the silicon nitride (SiN_(x)) film is etched byanisotropic etching can be used, for example. Thus, the silicon oxide(SiO_(x)) film or the silicon nitride (SiN_(x)) film remains only on theside surface of the gate electrode 110117, so that the sidewall 110121can be formed on the side surface of the gate electrode 110117.

FIG. 50 shows cross-sectional structures of a bottom gate transistor anda capacitor.

A first insulating film (an insulating film 110502) is formed entirelyover a substrate 110501. However, the first insulating film (theinsulating film 110502) may not be formed in some cases without beinglimited to this structure. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing a property of a transistor. That is, the firstinsulating film functions as a base film. Therefore, a highly reliabletransistor can be manufactured. As the first insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, or a silicon oxynitride film (SiO_(x)N_(y)) can be used.

A first conductive layer (a conductive layer 110503 and a conductivelayer 110504) is formed over the first insulating film. The conductivelayer 110503 includes a portion of a gate electrode of the transistor110520. The conductive layer 110504 includes a portion of a firstelectrode of a capacitor 110521. As the first conductive layer, Ti, Mo,Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or analloy of these elements can be used. Further, a stacked layer includingany of these (including an alloy thereof) can be used.

A second insulating film (an insulating film 110514) is formed to coverat least the first conductive layer. The second insulating film servesalso as a gate insulating film. As the second insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, or a silicon oxynitride film (SiO_(x)N_(y)) can be used.

As the second insulating film which is in contact with the semiconductorlayer, a silicon oxide film is preferably used. This is because the traplevels at the interface between the semiconductor layer and the secondinsulating film can be reduced.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as the second insulating film in contact withMo. This is because the silicon oxide film does not oxidize Mo.

A semiconductor layer is formed in a portion over the second insulatingfilm which overlaps with the first conductive layer by aphotolithography method, an inkjet method, a printing method or thelike. A portion of the semiconductor layer extends to a portion in whichthe second insulating film and the first conductive layer are notoverlapped and which is over the second insulating film. Thesemiconductor layer includes a channel region (a channel region 110510),LDD regions (an LDD region 110508 and an LDD region 110509), andimpurity regions (an impurity region 110505, an impurity region 110506,and an impurity region 110507). The channel region 110510 functions as achannel region of the transistor 110520. The LDD regions 110508 and110509 function as LDD regions of the transistor 110520. Note that theLDD regions 110508 and 110509 are not necessarily formed. The impurityregion 110505 includes one of a source electrode and a drain electrodeof the transistor 110520. The impurity region 110506 includes the otherof a source electrode and a drain electrode of the transistor 110520.The impurity region 110507 includes a second electrode of the capacitor110521.

A third insulating film (an insulating film 110511) is formed entirely.A contact hole is selectively formed in part of the third insulatingfilm. The insulating film 110511 has a function of an interlayerinsulating film. As the third insulating film, an inorganic material(e.g., silicon oxide (SiOx), silicon nitride, or silicon oxynitride), anorganic compound material having a low dielectric constant (e.g., aphotosensitive or nonphotosensitive organic resin material), or the likecan be used. Alternatively, a material including siloxane may be used.Siloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substituent, an organic groupincluding at least hydrogen (e.g., an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group can be used as thesubstituent. Further alternatively, the organic group including at leasthydrogen and the fluoro group may be used as the substituent.

A second conductive layer (a conductive layer 110512 and a conductivelayer 110513) is formed over the third insulating film. The conductivelayer 110512 is connected to the other of the source electrode and thedrain electrode of the transistor 110520 through the contact hole formedin the third insulating film. Therefore, the conductive layer 110512includes the other of the source electrode and the drain electrode ofthe transistor 110520. When the conductive layer 110513 is electricallyconnected to the conductive layer 110504, the conductive layer 11513includes a portion of a first electrode of the capacitor 110521.Alternatively, when the conductive layer 110513 is electricallyconnected to the impurity region 110507, the conductive layer 110513includes a portion of a second electrode of the capacitor 110521.Alternatively, when the conductive layer 110513 is connected to theconductive layer 110504 and the impurity region 110507, anothercapacitor is formed other than the capacitor 110521. In this capacitor,the conductive layer 110513, the impurity region 110507 and theinsulating layer 110511 are used as a first electrode, a secondelectrode and an insulating layer, respectively. Note that as the secondconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Further, astacked layer including any of these (including an alloy thereof) can beused.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

Next, structure of a transistor using amorphous silicon (a-Si) ormicrocrystal silicon as a semiconductor layer of the transistor and acapacitor are described.

FIG. 47 shows cross-sectional structures of a top gate transistor and acapacitor.

A first insulating film (an insulating film 110202) is formed entirelyover a substrate 110201. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing a property of a transistor. That is, the firstinsulating film functions as a base film. Therefore, a highly reliabletransistor can be manufactured. As the first insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, or a silicon oxynitride film (SiO_(x)N_(y)) can be used.

The first insulating film is not necessarily formed. If the firstinsulating film is not formed, the number of steps can be reduced, andthe manufacturing cost can be reduced. Since the structure can besimplified, yield can be increased.

A first conductive layer (a conductive layer 110203, a conductive layer110204, and a conductive layer 110205) is formed over the firstinsulating film. The conductive layer 110203 includes a portion of oneof a source electrode and a drain electrode of a transistor 110220. Theconductive layer 110204 includes a portion of the other of a sourceelectrode and a drain electrode of the transistor 110220. The conductivelayer 110205 includes a portion of a first electrode of a capacitor110221. As the first conductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu,Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements canbe used. Further, a stacked layer including any of these (including analloy thereof) can be used.

Over the conductive layer 110203 and the conductive layer 110204, afirst semiconductor layer (a semiconductor layer 110206 and asemiconductor layer 110207) is formed. The semiconductor layer 110206includes a portion of one of a source electrode and a drain electrode.The semiconductor layer 110207 includes a portion of the other of thesource electrode and the drain electrode. As the first semiconductorlayer, silicon including phosphorus or the like can be used.

A second semiconductor layer (a semiconductor layer 110208) is formedbetween the conductive layer 110203 and the conductive layer 110204, andover the first insulating film. A part of the semiconductor layer 110208extends to a portion over the conductive layer 110203 and the conductivelayer 110204. The semiconductor layer 110208 includes a portion of achannel region of the transistor 110220. As the second semiconductorlayer, a semiconductor layer having non-crystallinity such as amorphoussilicon (a-Si:H), or a semiconductor layer such as microcrystal (μ-Si:H)can be used.

A second insulating film (an insulating film 110209 and an insulatingfilm 110210) is formed to cover at least the semiconductor layer 110208and the conductive layer 110205. The second insulating film serves alsoas a gate insulating film. As the second insulating film, a single layeror a stacked layer of a silicon oxide film, a silicon nitride film, or asilicon oxynitride film (SiO_(x)N_(y)) can be used.

As the second insulating film which is in contact with the secondsemiconductor layer, a silicon oxide film is preferably used. This isbecause the trap levels at the interface between the secondsemiconductor layer and the second insulating film can be reduced.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as the second insulating film in contact withMo. This is because the silicon oxide film does not oxidize Mo.

A second conductive layer (a conductive layer 110211 and a conductivelayer 110212) is formed over the second insulating film. The conductivelayer 110211 includes a portion of a gate electrode of the transistor110220. The conductive layer 110212 includes a portion of a secondelectrode or a wiring of a capacitor 110221. As the second conductivelayer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, orGe, or an alloy of these elements can be used. Further, a stacked layerincluding any of these (including an alloy thereof) can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

FIG. 48 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorillustrated in FIG. 48 is a channel-etched type transistor.

A first insulating film (an insulating film 110302) is formed entirelyover a substrate 110301. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing a property of the transistor. That is, the firstinsulating film functions as a base film. Therefore, a highly reliabletransistor can be manufactured. As the first insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, or a silicon oxynitride film (SiO_(x)N_(y)) can be used.

The first insulating film is not necessarily formed. If the firstinsulating film is not formed, the number of steps can be reduced, andthe manufacturing cost can be reduced. Since the structure can besimplified, yield can be increased.

A first conductive layer (a conductive layer 110303 and a conductivelayer 110304) is formed over the first insulating film. The conductivelayer 110303 includes a portion of a gate electrode of the transistor110320. The conductive layer 110304 includes a portion of a firstelectrode of a capacitor 110321. As the first conductive layer, Ti, Mo,Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or analloy of these elements can be used. Further, a stacked layer includingany of these (including an alloy thereof) can be used.

A second insulating film (an insulating film 110305) is formed so as tocover at least the first conductive layer. The second insulating filmserves also as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, or a silicon oxynitride film (SiO_(x)N_(y)) can be used.

As the second insulating film which is in contact with the semiconductorlayer, a silicon oxide film is preferably used. This is because the traplevels at the interface between the semiconductor layer and the secondinsulating film can be reduced.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as the second insulating film in contact withMo. This is because the silicon oxide film does not oxidize Mo.

A first semiconductor layer (a semiconductor layer 110306) is formed ina portion over the second insulating film which overlaps with the firstconductive layer by a photolithography method, an inkjet method, aprinting method or the like. A portion of the semiconductor layer 110306extends to a portion in which the second insulating film and the firstconductive layer are not overlapped. The semiconductor layer 110306includes a portion of a channel region of the transistor 110320. As thesemiconductor layer 110306, a semiconductor layer havingnon-crystallinity such as amorphous silicon (a-Si:H), or a semiconductorlayer such as microcrystal (μ-Si:H) can be used.

In a portion over the first semiconductor layer, a second semiconductorlayer (a semiconductor layer 110307 and a semiconductor layer 110308) isformed. The semiconductor layer 110307 includes a portion of one of asource electrode and a drain electrode. The semiconductor layer 110308includes a portion of the other of the source electrode and the drainelectrode. As the second semiconductor layer, silicon includingphosphorus or the like can be used.

A second conductive layer (a conductive layer 110309, a conductive layer110310, and a conductive layer 110311) is formed over the secondsemiconductor layer and the second insulating film. The conductive layer110309 includes a portion of one of a source electrode and a drainelectrode of the transistor 110320. The conductive layer 110310 includesthe other of the source electrode and the drain electrode of thetransistor 110320. The conductive layer 110311 includes a portion of asecond electrode of the capacitor 110321. Note that as the secondconductive layer, Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Further, astacked layer including any of these (including an alloy thereof) can beused.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

A process of forming a channel-etched type transistor is described as anexample. The first semiconductor layer and the second semiconductorlayer can be formed using the same mask. Specifically, the firstsemiconductor layer and the second semiconductor layer are formedsequentially. The first semiconductor layer and the second semiconductorlayer are formed using the same mask.

A process of forming a channel-etched type transistor is described asanother example. Without using a new mask, a channel region of atransistor is formed. Specifically, after forming the second conductivelayer, a part of the second semiconductor layer is removed using thesecond conductive layer as a mask. Alternatively, a portion of thesecond semiconductor layer is removed by using the same mask as thesecond conductive layer. The first semiconductor layer below the removedsecond semiconductor layer becomes a channel region of the transistor.

FIG. 49 illustrates cross-sectional structures of an inversely staggered(a bottom gate) transistor and a capacitor. In particular, thetransistor illustrated in FIG. 49 is a channel protection (a channelstop) type transistor.

A first insulating film (an insulating film 110402) is formed entirelyover a substrate 110401. The first insulating film can preventimpurities from the substrate from adversely affecting a semiconductorlayer and changing a property of a transistor. That is, the firstinsulating film functions as a base film. Therefore, a highly reliabletransistor can be manufactured. As the first insulating film, a singlelayer or a stacked layer of a silicon oxide film, a silicon nitridefilm, or a silicon oxynitride film (SiO_(x)N_(y)) can be used.

The first insulating film is not necessarily formed. If the firstinsulating film is not formed, the number of steps can be reduced, andthe manufacturing cost can be reduced. Since the structure can besimplified, yield can be increased.

A first conductive layer (a conductive layer 110403 and a conductivelayer 110404) is formed over the first insulating film. The conductivelayer 110403 includes a portion of a gate electrode of a transistor110420. The conductive layer 110404 includes a portion of a firstelectrode of a capacitor 110421. As the first conductive layer, Ti, Mo,Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or analloy of these elements can be used. Further, a stacked layer includingany of these (including an alloy thereof) can be used.

A second insulating film (an insulating film 110405) is formed so as tocover at least the first conductive layer. The second insulating filmserves also as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, or a silicon oxynitride film (SiO_(x)N_(y)) can be used.

As the second insulating film which is in contact with the semiconductorlayer, a silicon oxide film is preferably used. This is because the traplevels at the interface between the semiconductor layer and the secondinsulating film can be reduced.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used as the second insulating film in contact withMo. This is because the silicon oxide film does not oxidize Mo.

A first semiconductor layer (a semiconductor layer 110406) is formed ina portion over the second insulating film which overlaps with the firstconductive layer, by a photolithography method, an inkjet method, aprinting method or the like. A portion of the semiconductor layer 110406extends to a portion in which the second insulating film and the firstconductive layer are not overlapped. The semiconductor layer 110406includes a portion of a channel region of the transistor r110420. As thesemiconductor layer 110406, a semiconductor layer havingnon-crystallinity such as amorphous silicon (a-Si:H), or a semiconductorlayer such as microcrystal (μ-Si:H) can be used, for example.

A third insulating film (an insulating film 110412) is formed in aportion over the first semiconductor layer. The insulating film 110412has a function of preventing the channel region of the transistor 110420from being etched. That is, the insulating film 110412 functions as achannel protection film (a channel stop film). As the third insulatingfilm, a single layer or a stacked layer of a silicon oxide film, asilicon nitride film, or a silicon oxynitride film (SiO_(x)N_(y)) can beused.

In a portion over the first semiconductor layer and a portion over thethird insulating film, a second semiconductor layer (a semiconductorlayer 110407 and a semiconductor layer 110408) is formed. Thesemiconductor layer 110407 includes a portion of one of a sourceelectrode and a drain electrode. The semiconductor layer 110408 includesa portion of the other of the source electrode and the drain electrode.As the second semiconductor layer, silicon including phosphorus or thelike can be used.

A second conductive layer (a conductive layer 110409, a conductive layer110410, and a conductive layer 110411) is formed over the secondsemiconductor layer. The conductive layer 110409 includes a portion ofone of a source electrode and a drain electrode of the transistor110420. The conductive layer 110410 includes the other of the sourceelectrode and the drain electrode of the transistor 110420. Theconductive layer 110411 includes a portion of a second electrode of thecapacitor 110421. Note that as the second conductive layer, Ti, Mo, Ta,Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy ofthese elements can be used. Further, a stacked layer including any ofthese (including an alloy thereof) can be used.

In steps after forming the second conductive layer, various insulatingfilms or various conductive films may be formed.

The structures and manufacturing methods of such transistors have beendescribed above. Such wirings, electrodes, conductive layers, conductivefilms, terminals, bias or plugs are formed to have one or more elementsselected from the group consisting of aluminum (Al), tantalum (Ta),titanium (Ti), molybdenum (Mo), tungsten (W), neodymium (Nd), chromium(Cr), nickel (Ni), platinum (Pt), gold (Au), silver (Ag), copper (Cu),magnesium (Mg), scandium (Sc), cobalt (Co), zinc (Zn), niobium (Nb),silicon (Si), phosphorus (P), boron (B), arsenic (As), gallium (Ga),indium (In), tin (Sn), and oxygen (O); a compound or an alloy materialincluding one or more of the elements in the group (for example, indiumtin oxide (ITO), indium zinc oxide (IZO), indium tin oxide to whichsilicon oxide is added (ITSO), zinc oxide (ZnO), tin oxide (Son),Cadmium tin oxide (CTO), aluminum neodymium (Al—Nd), magnesium silver(Mg—Ag), molybdenum-niobium (Mo—Nb) or the like); a substance in whichthese compounds are combined; or the like. Alternatively, such wirings,electrodes, conductive layers, conductive films, terminals arepreferably formed to have a substance including such compounds, acompound of silicon and one or more of the elements selected from thegroup (silicide) (e.g., aluminum silicon, molybdenum silicon, nickelsilicide); or a compound of nitrogen and one or more of the elementsselected from the group (e.g., titanium nitride, tantalum nitride,molybdenum nitride).

Note that silicon (Si) may include an n-type impurity (e.g., phosphorus)or a p-type impurity (e.g., boron). The impurity contained in siliconcan increase the conductivity or enables the same performance as normalconductors. Thus, such silicon can be utilized easily as wirings orelectrodes.

Silicon can be any of various types of silicon such as singlecrystalline silicon, polycrystal silicon, or microcrystal silicon.Alternatively, silicon having no crystallinity such as amorphous siliconcan be used. By using single crystalline silicon or polycrystal silicon,resistance of a wiring, an electrode, a conductive layer, a conductivefilm, or a terminal can be reduced. By using amorphous silicon or microcrystalline silicon, a wiring or the like can be formed by a simpleprocess.

In addition, aluminum or silver has high conductivity, and thus canreduce a signal delay. Since aluminum or silver can be easily etched,aluminum or silver can be easily patterned and processed minutely.

Further, copper has also high conductivity, and thus can reduce a signaldelay. In using copper, a stacked structure is preferably employed sincecopper increases the adhesion.

Molybdenum and titanium are also preferable materials. This is becauseeven if molybdenum or titanium is in contact with an oxide of asemiconductor (e.g., ITO or IZO) or silicon, molybdenum or titanium doesnot cause defects. Further, molybdenum or titanium is easily etched andhas high-heat resistance.

Tungsten is preferable since tungsten has high-heat resistance.

Neodymium is also preferable, since neodymium has an advantage of highheat resistance. In particular, an alloy of neodymium and aluminum isused to increase heat-resistance, thereby almost preventing hillocks ofaluminum.

Moreover, silicon is preferable since silicon can be formed at the sametime as a semiconductor layer included in a transistor, and hashigh-heat resistance.

Since ITO, IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO),and cadmium tin oxide (CTO) have light-transmitting properties, they canbe used as a portion which light should pass through. For example, ITO,IZO, ITSO, zinc oxide (ZnO), silicon (Si), tin oxide (SnO), or cadmiumtin oxide (CTO) can be used for a pixel electrode or a common electrode.

IZO is preferable since IZO is easily etched and processed. In etchingIZO, almost no residues of IZO are left. Thus, when a pixel electrode isformed using IZO, defects (such as short-circuiting or orientationdisorder) of a liquid crystal element or a light-emitting element can bereduced.

Such wirings, electrodes, conductive layers, conductive films,terminals, via holes, or plugs may have a single-layer structure or amultilayer structure. By adopting a single-layer structure, amanufacturing process of such wirings, electrodes, conductive layers,conductive films, or terminals can be simplified; the number of days fora process can be reduced; and cost can be reduced. Alternatively, byemploying a multilayer structure, an advantage of each material is takenand a disadvantage thereof is reduced so that a wiring or an electrodewith high performance can be formed. For example, a low-resistantmaterial (e.g., aluminum) is included in a multilayer structure, therebyreducing the resistance of such wirings. As another example, when a lowheat-resistant material is interposed between high heat-resistantmaterials to form a stacked-layer structure, heat resistance of wiringsor electrodes can be increased, utilizing advantages of such lowheat-resistance materials. For example, a layer including aluminum ispreferably interposed between layers including molybdenum, titanium, orneodymium as a stacked structure.

If wirings or electrodes are in direct contact with each other, anadverse effect is caused to each other in some cases. For example, oneof a wiring and an electrode is mixed into another of the wirings orelectrodes and changes the property, and thus, a desired function cannotbe obtained. As another example, in forming a high-resistant portion,there is a problem in that it cannot be formed normally. In such a base,a reactive material is preferably sandwiched by or covered with anon-reactive material in a stacked structure. For example, when ITO isconnected to aluminum, an alloy of titanium, molybdenum, and neodymiumis preferably disposed between the ITO and the aluminum. As anotherexample, when silicon is connected to aluminum, an alloy of titanium,molybdenum, and neodymium is preferably disposed between the silicon andthe aluminum.

Note that the term “wiring” indicates a portion including a conductor.The shape of such a wiring may be linear; but not limited to, such awiring may be short. Therefore, electrodes are included in such wirings.

Note that a carbon nanotube may be used for wirings, electrodes,conductive layers, conductive films, terminals, via holes, or plugs.Since the carbon nanotube has a light-transmitting property, it can beused for a portion which light should pass thorough. For example, thecarbon nanotube can be used for a pixel electrode and/or a commonelectrode.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied ease ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 9

In this embodiment mode, a structure of a display device is described.

A structure of a display device is described with reference to FIG. 53A.FIG. 53A is a top plan view of the display device.

A pixel portion 170101, a scan line side input terminal 170103, and asignal line side input terminal 170104 are formed over a substrate170100, scan lines extend in a row direction from the scan line sideinput terminal 170103, and signal lines extend in a column directionfrom the signal line side input terminal 170104 over the substrate170100. Pixels are arranged in matrix and each pixel 170102 is arrangedat an intersection of the scan line and the signal line in the pixelportion 170101.

The case in which signals are input from an external driver circuit hasbeen described above. However, the present invention is not limited tothis, and an IC chip can be mounted on the display device.

For example, as shown in FIG. 54A, an IC chip 170201 can be mounted on asubstrate 170100 by a COG (Chip On Glass) method. In this case,inspection can be conducted before mounting the IC chip 170201 on thesubstrate 170100 to increase yield of the display device. Further,reliability can also increase. In addition, portions which are common tothose in FIG. 53A are denoted by common reference numerals anddescription thereof is omitted.

As another example, as shown in FIG. 54B, the IC chip 170201 can bemounted on an FPC (Flexible Printed Circuit) 170200 by a TAB (TapeAutomated Bonding) method. In this case, inspection can be conductedbefore mounting the IC chip 170201 on the FPC 170200 to increase yieldof the display device. Further, reliability can also increase. Inaddition, portions which are common to those in FIG. 53A are denoted bycommon reference numerals and description thereof is omitted.

As well as the IC chip can be mounted on the substrate 170100, a drivercircuit can be mounted on the substrate 170100.

For example, as shown in FIG. 53B, a scan line driver circuit 170105 canbe formed on the substrate 170100. In this case, the number of componentparts can be reduced to decrease manufacturing cost. The number ofconnection points between component parts can be reduced to improvereliability. Since driving frequency of the scan line driver circuit170105 is low, the scan line driver circuit 170105 can be easily formedusing amorphous silicon or microcrystal silicon as a semiconductor layerof a transistor. In addition, an IC chip for outputting a signal to thesignal line may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC to which an IC chip for outputting a signal to asignal line is mounted by a TAB method may be arranged on the substrate170100. In addition, an IC chip for controlling the scan line drivercircuit 170105 may be mounted on the substrate 170100 by a COG method.Alternatively, an FPC to which an IC chip for controlling the scan linedriver circuit 170105 is mounted by a TAB method may be disposed on thesubstrate 170100. In addition, portions which are common to those inFIG. 53A are denoted by common reference numerals and descriptionthereof is omitted.

As another example, as shown in FIG. 53C, the scan line driver circuit170105 and the signal line driver circuit 170106 are formed over thesubstrate 170100. Thus, the number of component parts can be reduced todecrease manufacturing cost. The number of connection points betweencomponent parts can be reduced to improve reliability. In addition, theIC chip for controlling the scan line driver circuit 170105 may bemounted on the substrate 170100 by a COG method. Alternatively, the FPCto which an IC chip for controlling the scan line driver circuit 170105is mounted by a TAB method may be arranged on the substrate 170100. AnIC chip for controlling the signal line driver circuit 170106 may bemounted on the substrate 170100 by a COG method. Alternatively, an ICchip for controlling the signal line driver circuit 170106 may bemounted on the substrate 170100 by a TAB method. In addition, portionswhich are common to those in FIG. 53A are denoted by common referencenumerals and description thereof is omitted.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 10

In this embodiment mode, a method for driving a display device isdescribed. In particular, a method for driving a liquid crystal displaydevice is described.

A liquid crystal display panel which can be used for the liquid crystaldisplay device described in this embodiment mode has a structure inwhich a liquid crystal material is sandwiched between two substrates. Anelectrode for controlling an electric field applied to the liquidcrystal material is provided in each of the two substrates. A liquidcrystal material corresponds to a material the optical and electricalproperties of which is changed by an electric field applied fromoutside. Therefore, a liquid crystal panel corresponds to a device inwhich desired optical and electrical properties can be obtained bycontrolling voltage applied to the liquid crystal material using theelectrode included in each of the two substrates. In addition, a largenumber of electrodes are arranged in a planar manner, each of theelectrodes corresponds to a pixel, and voltages applied to the pixelsare individually controlled. Therefore, a liquid crystal display panelwhich can display a clear image can be obtained.

Here, response time of the liquid crystal material with respect tochange in an electric field depends on a gap between the two substrates(a cell gap) and a type or the like of the liquid crystal material, andis generally several milli-seconds to several ten milli-seconds.Further, in the case where the amount of change in the electric field issmall, the response time of the liquid crystal material is furtherlengthened. This characteristic causes a defect in image display such asan after image, a phenomenon in which traces can be seen, or decrease incontrast when the liquid crystal panel displays a moving image. Inparticular, when a half tone is changed into another half tone (changein the electric field is small), a degree of the above-described defectbecomes noticeable.

Meanwhile, as a particular problem of a liquid crystal panel using anactive matrix method, fluctuation in writing voltage due to constantelectric charge driving is given. Constant electric charge driving inthis embodiment mode is described below.

A pixel circuit using an active matrix method includes a switch whichcontrols writing and a capacitor which holds an electric charge. Amethod for driving the pixel circuit using the active matrix methodcorresponds to a method in which predetermined voltage is written in apixel circuit with a switch in an on state, and immediately after that,an electric charge in the pixel circuit is held (a hold state) with theswitch in an off state. At the time of hold state, exchange of theelectric charge between inside and outside of the pixel circuit is notperformed (a constant electric charge). Usually, a period in which theswitch is in an off state is approximately several hundreds of times(the number of scan lines) longer than a period in which the switch isin an on state. Therefore, it may be considered that the switch of thepixel circuit be almost always in an off state. As described above,constant electric charge driving in this embodiment mode corresponds toa driving method in which a pixel circuit is in a hold state in almostall periods in driving a liquid crystal panel.

Next, electrical properties of the liquid crystal material aredescribed. A dielectric constant as well as optical properties of theliquid crystal material are changed when an electric field applied fromoutside is changed. That is, when it is considered that each pixel ofthe liquid crystal panel be a capacitor (a liquid crystal element)sandwiched between two electrodes, the capacitor corresponds to acapacitor, capacitance of which is changed in accordance with appliedvoltage. This phenomenon is called dynamic capacitance.

When a capacitor, capacitance of which is changed in accordance withapplied voltage in this manner is driven by constant electric chargedriving, the following problem occurs. When capacitance of a liquidcrystal element is changed in a hold state in which an electric chargeis not moved, applied voltage is also changed. This is not difficult tounderstand from the fact that the amount of electric charges is constantin a relational expression of (the amount of electriccharges)=(capacitance)×(applied voltage).

Because of the above-described reasons, voltage at the time of a holdstate is changed from voltage at the time of writing because constantelectric charge driving is performed in a liquid crystal panel using anactive matrix method. Accordingly, change in transmittivity of theliquid crystal element is different from change in transmittivity of aliquid crystal element in a driving method which does not take a holdstate. FIGS. 51A to 51C show this state. FIG. 51A shows an example ofcontrolling voltage written in a pixel circuit in the case where time isrepresented by a horizontal axis and the transmittivity of the liquidcrystal element is represented by a vertical axis. FIG. 51B shows anexample of controlling voltage written in the pixel circuit in the casewhere time is represented by a horizontal axis and the voltage isrepresented by a vertical axis. FIG. 51C shows time change intransmittivity of the liquid crystal element in the case where thevoltage shown in FIG. 51A or 51B is written in the pixel circuit whentime is represented by a horizontal axis and the absolute value of thevoltage is represented by a vertical axis. In each of FIGS. 51A to 51C,a period F shows a period for rewriting the voltage and time forrewriting the voltage is described as t₁, t₂, t₃, and t₄.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of 0and corresponds to |V₂| in rewriting at the time of t₁, t₂, t₃, and t₄(see FIG. 51A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 51B). Since direct voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note also that a period of switchingthe polarity (an inversion period) may be the same as a period ofrewriting voltage. In this case, generation of a flicker caused byinversion driving can be reduced because the inversion period is short.Further, the inversion period may be a period which is integral times ofthe period of rewriting voltage. In this case, power consumption can bereduced because the inversion period is long and frequency of writingvoltage can be decreased by changing the polarity.

FIG. 51C shows time change in transmittivity of the liquid crystalelement in the case where voltage as shown in FIG. 51A or 51B is appliedto the liquid crystal element. Here, the voltage |V₁| is applied to theliquid crystal element and transmittivity of the liquid crystal elementafter time passes sufficiently corresponds to TR₁. Similarly, thevoltage |V₂| is applied to the liquid crystal element and transmittivityof the liquid crystal element after time passes sufficiently correspondsto TR₂. When the voltage applied to the liquid crystal element ischanged from |V₁| to |V₂| at the time of t₁, transmittivity of theliquid crystal element does not immediately become TR₂ as shown by adashed line 30401 but slowly changes. For example, when the period ofrewriting voltage is the same as a frame period of an image signal of 60Hz (16.7 milli-seconds), time for several frames is necessary untiltransmittivity is changed to TR₂.

Note that smooth time change in transmittivity as shown in the dashedline 30401 corresponds to time change in transmittivity when the voltage|V₂| is accurately applied to the liquid crystal element. In an actualliquid crystal panel, for example, a liquid crystal panel using anactive matrix method, transmittivity of the liquid crystal does not havetime change as shown by the dashed line 30401 but has gradual timechange as shown by a solid line 30402 because voltage at the time of ahold state is changed from voltage at the time of writing due toconstant electric charge driving. This is because the voltage is changeddue to constant electric charge driving, so that it is impossible toreach intended voltage only by one writing. Accordingly, the responsetime of transmittivity of the liquid crystal element becomes furtherlonger than original response time (the dashed line 30401) inappearance, so that a defect in image display such as an after image, aphenomenon in which traces can be seen, or decrease in contrast occurs.

By using overdriving, it is possible to solve a phenomenon in which theresponse time in appearance becomes further longer because of shortageof writing by dynamic capacitance and constant electric charge drivingas well as length of the original response time of the liquid crystalelement. FIGS. 52A to 52C show this state. FIG. 52A shows an example ofcontrolling voltage written in a pixel circuit in the case where time isrepresented by a horizontal axis and the absolute value of the voltageis represented by a vertical axis. FIG. 52B shows an example ofcontrolling voltage written in the pixel circuit in the case where timeis represented by a horizontal axis and the voltage is represented by avertical axis. FIG. 52C shows time change in transmittivity of theliquid crystal element in the case where the voltage shown in FIG. 52Aor 52B is written in the pixel circuit when time is represented by ahorizontal axis and the absolute value of the voltage is represented bya vertical axis. In each of FIGS. 52A to 52C, a period F shows a periodfor rewriting the voltage and time for rewriting the voltage isdescribed as t₁, t₂, t₃, and t₄.

Here, writing voltage corresponding to image data input to the liquidcrystal display device corresponds to |V₁| in rewriting at the time of0, corresponds to |V₃| in rewriting at the time of t₁, and correspondsto |V₃| in writing at the time of t₂, t₃, and t₄ (see FIG. 52A).

Note that polarity of the writing voltage corresponding to image datainput to the liquid crystal display device may be switched periodically(inversion driving: see FIG. 52B). Since direct voltage can be preventedfrom being applied to a liquid crystal as much as possible by using thismethod, burn-in or the like caused by deterioration of the liquidcrystal element can be prevented. Note also that a period of switchingthe polarity (an inversion period) may be the same as a period ofrewriting voltage. In this case, generation of a flicker caused byinversion driving can be reduced because the inversion period is short.Further, the inversion period may be a period which is integral times ofthe period of rewriting voltage. In this case, power consumption can bereduced because the inversion period is long and frequency of writingvoltage can be decreased by changing the polarity.

FIG. 52C shows time change in transmittivity of the liquid crystalelement in the case where voltage as shown in FIG. 52A or 52B is appliedto the liquid crystal element. Here, the voltage |V₁| is applied to theliquid crystal element and transmittivity of the liquid crystal elementafter time passes sufficiently corresponds to TR₁. Similarly, thevoltage |V₂| is applied to the liquid crystal element and transmittivityof the liquid crystal element after time passes sufficiently correspondsto TR₂. Similarly, the voltage |V₃| is applied to the liquid crystalelement and transmittivity of the liquid crystal element after timepasses sufficiently corresponds to TR₃. When the voltage applied to theliquid crystal element is changed from |V₁| to |V₃| at the time of t₁,transmittivity of the liquid crystal element is tried to be changed toTR₃ for several frames as shown by a dashed line 30501. However,application of the voltage |V₃| is terminated at the time t₂ and thevoltage |V₂| is applied after the time t₂. Therefore, transmittivity ofthe liquid crystal element does not become as shown by the dashed line30501 but becomes as shown by a solid line 30502. Here, it is preferablethat a value of the voltage |V₃| be set so that transmittivity isapproximately TR₂ at the time of t₂. Here, the voltage |V₃| is alsoreferred to as overdriving voltage.

That is, the response time of the liquid crystal element can becontrolled to some extent by changing |V₃| which is the overdrivingvoltage. This is because the response time of the liquid crystal elementis changed by strength of an electric field. Specifically, the responsetime of the liquid crystal element becomes shorter as the electric fieldis strong, and the response time of the liquid crystal element becomeslonger as the electric field is weak.

Note that it is preferable that |V₃| which is the overdriving voltage bechanged in accordance with the amount of change in the voltage, i.e.,the voltage |V₁| and the voltage |V₂| which supply intendedtransmittivity TR₁ and TR₂. This is because appropriate response timecan be always obtained by changing |V₃| which is the overdriving voltagein accordance with change in the response time of the liquid crystalelement even when the response time of the liquid crystal element ischanged by the amount of change in the voltage.

Note also that it is preferable that |V₃| which is the overdrivingvoltage be changed by a mode of the liquid crystal element such as a TNmode, a VA mode, an IPS mode, or an OCB mode. This is becauseappropriate response time can be always obtained by changing |V₃| whichis the overdriving voltage in accordance with change in the responsetime of the liquid crystal element even when the response time of theliquid crystal element is changed by the mode of the liquid crystalelement.

Note also that the voltage rewriting period F may be the same as a frameperiod of an input signal. In this case, a liquid crystal display devicewith low manufacturing cost can be obtained because a peripheral drivercircuit of the liquid crystal display device can be simplified.

Note also that the voltage rewriting period F may be shorter than theframe period of the input signal. For example, the voltage rewritingperiod F may be one half the frame period of the input signal, one thirdthe frame period of the input signal, or one third or less the frameperiod of the input signal. It is effective to combine this method witha countermeasure against deterioration in quality of a moving imagecaused by hold driving of the liquid crystal display device such asblack data insertion driving, backlight blinking, backlight scanning, orintermediate image insertion driving by motion compensation. That is,since required response time of the liquid crystal element is short inthe countermeasure against deterioration in quality of a moving imagecaused by hold driving of the liquid crystal display device, theresponse time of the liquid crystal element can be relatively shortenedeasily by using overdriving described in this embodiment mode. Althoughthe response time of the liquid crystal element can be essentiallyshortened by a cell gap, a liquid crystal material, a mode of the liquidcrystal element, or the like, it is technically difficult to shorten theresponse time of the liquid crystal element. Therefore, it is veryimportant to use a method for shortening the response time of the liquidcrystal element by a driving method such as overdriving.

Note also that the voltage rewriting period F may be longer than theframe period of the input signal. For example, the voltage rewritingperiod F may be twice the frame period of the input signal, three timesthe frame period of the input signal, or three times or more the frameperiod of the input signal. It is effective to combine this method witha unit (a circuit) which determines whether voltage is not rewritten fora long period or not. That is, when the voltage is not rewritten for along period, an operation of the circuit can be stopped during a periodwhere no voltage is rewritten without performing a rewriting operationitself of the voltage. Therefore, a liquid crystal display device withlow power consumption can be obtained.

Next, a specific method for changing |V₃| which is the overdrivingvoltage in accordance with the voltage |V₁| and the voltage |V₂| whichsupply intended transmittivity TR₁ and TR₂ is described.

Since an overdriving circuit corresponds to a circuit for appropriatelycontrolling |V₃| which is the overdriving voltage in accordance with thevoltage |V₁| and the voltage |V₂| which supply intended transmittivityTR₁ and TR₂, signals input to the overdriving circuit are a signal whichis related to the voltage |V₁| which supplies intended transmittivityTR₁ and a signal which is related to the voltage |V₂| which suppliesintended transmittivity TR₂, and a signal output from the overdrivingcircuit is a signal which is related to |V₃| which is the overdrivingvoltage. Here, each of these signals may have an analog voltage valuesuch as the voltage applied to the liquid crystal element (e.g., |V₁|,|V₁|, or |V₃|) or may be a digital signal for supplying the voltageapplied to the liquid crystal element. Here, the signal which is relatedto the overdriving circuit is described as a digital signal.

First, a general structure of the overdriving circuit is described withreference to FIG. 88A. Here, input image signals 30101 a and 30101 b areused as signals for controlling the overdriving voltage. As a result ofprocessing these signals, an output image signal 30104 is to be outputas a signal which supplies the overdriving voltage.

Here, since the voltage |V₁| and the voltage |V₂| which supply intendedtransmittivity TR₁ and TR₂ are image signals in adjacent frames, it ispreferable that the input image signals 30101 a and 30101 b be similarlyimage signals in adjacent frames. In order to obtain such signals, theinput image signal 30101 a is input to a delay circuit 30102 in FIG. 88Aand a signal which is consequently output can be used as the input imagesignal 30101 b. For example, a memory can be given as the delay circuit30102. That is, the input image signal 30101 a is stored in the memoryin order to delay the input image signal 30101 a for one frame; a signalstored in the previous frame is taken out from the memory as the inputimage signal 30101 b at the same time; and the input image signal 30101a and the input image signal 30101 b are simultaneously input to acorrection circuit 30103. Therefore, the image signals in adjacentframes can be handled. By inputting the image signals in adjacent framesto the correction circuit 30103, the output image signal 30104 can beobtained. Note that when a memory is used as the delay circuit 30102, amemory having capacity for storing an image signal for one frame inorder to delay the input image signal 30101 a for one frame (i.e., aframe memory) can be obtained. Thus, the memory can have a function as adelay circuit without causing excess and deficiency of memory capacity.

Next, the delay circuit 30102 formed mainly for reducing memory capacityis described. Since memory capacity can be reduced by using such acircuit as the delay circuit 30102, manufacturing cost can be reduced.

Specifically, a delay circuit as shown in FIG. 88B can be used as thedelay circuit 30102 having such characteristics. The delay circuit shownin FIG. 88B includes an encoder 30105, a memory 30106, and a decoder30107.

Operations of the delay circuit 30102 shown in FIG. 88B are as follows.First, compression treatment is performed by the encoder 30105 beforethe input image signal 30101 a is stored in the memory 30106. Thus, sizeof data to be stored in the memory 30106 can be reduced. Accordingly,since memory capacity can be reduced, manufacturing cost can also bereduced. Then, a compressed image signal is transferred to the decoder30107 and extension treatment is performed here. Thus, the previoussignal which is compressed by the encoder 30105 can be restored. Here,compression and extension treatment which is performed by the encoder30105 and the decoder 30107 may be reversible treatment. Thus, since theimage signal does not deteriorate even after compression and extensiontreatment is performed, memory capacity can be reduced without causingdeterioration of quality of an image, which is finally displayed on adevice. Further, compression and extension treatment which is performedby the encoder 30105 and the decoder 30107 may be non-reversibletreatment. Thus, since size of data of the compressed image signal canbe extremely made small, memory capacity can be significantly reduced.

Note that as a method for reducing memory capacity, various methods canbe used as well as the above-described method. A method in which colorinformation included in an image signal is reduced (e.g., tone reductionfrom 2.6 hundred thousand colors to 65 thousand colors is performed) orthe amount of data is reduced (e.g., resolution is made small) withoutperforming image compression by an encoder, or the like can be used.

Next, specific examples of the correction circuit 30103 are describedwith reference to FIGS. 88C to 88E. The correction circuit 30103corresponds to a circuit for outputting an output image signal having acertain value from two input image signals. Here, when relation betweenthe two input image signals and the output image signal is non-linearand it is difficult to calculate the relation by simple operation, alook up table (an LUT) may be used as the correction circuit 30103.Since the relation between the two input image signals and the outputimage signal is calculated in advance by measurement in an LUT, theoutput image signal corresponding to the two input image signals can becalculated only by seeing the LUT (see FIG. 88C). By using a LUT 30108as the correction circuit 30103, the correction circuit 30103 can berealized without performing complicated circuit design or the like.

Here, since the LUT 30108 is one of memories, it is preferable to reducememory capacity as much as possible in order to reduce manufacturingcost. As an example of the correction circuit 30103 for realizingreduction in memory capacity, a circuit shown in FIG. 88D can be given.The correction circuit 30103 shown in FIG. 88D includes an LUT 30109 andan adder 30110. Data of difference between the input image signal 30101a and the output image signal 30104 to be output is stored in the LUT30109. That is, corresponding difference data from the input imagesignal 30101 a and the input image signal 30101 b is taken out from theLUT 30109 and taken out difference data and the input image signal 30101a are added by the adder 30110, so that the output image signal 30104can be obtained. Note that when data stored in the LUT 30109 isdifference data, memory capacity of the LUT 30109 can be reduced. Thisis because data size of difference data is smaller than data size of theoutput image signal 30104 itself, so that memory capacity necessary forthe LUT 30109 can be made small.

In addition, when the output image signal can be calculated by simpleoperation such as four arithmetic operations of the two input imagesignals, the correction circuit 30103 can be realized by combination ofsimple circuits such as an adder, a subtracter, and a multiplier.Accordingly, it is not necessary to use a LUT, so that manufacturingcost can be significantly reduced. As such a circuit, a circuit shown inFIG. 88E can be given. The correction circuit 30103 shown in FIG. 88Eincludes a subtracter 30111, a multiplier 30112, and an adder 30113.First, difference between the input image signal 30101 a and the inputimage signal 30101 b is calculated by the subtracter 30111. After that,a differential value is multiplied by an appropriate coefficient byusing the multiplier 30112. Then, by adding the differential valuemultiplied by appropriate coefficient to the input image signal 30101 aby the adder 30113, the output image signal 30104 can be obtained. Byusing such a circuit, it is not necessary to use the LUT. Therefore,manufacturing cost can be significantly reduced.

Note that by using the correction circuit 30103 shown in FIG. 88E undera certain condition, output of the inappropriate output image signal30104 can be prevented. The condition is as follows. The output imagesignal 30104 applying the overdriving voltage and a differential valuebetween the input image signals 30101 a and 30101 b have linearity. Inaddition, the differential value corresponds to a coefficient multipliedby inclination of this linearity by using the multiplier 30112. That is,it is preferable that the correction circuit 30103 shown in FIG. 88E beused for a liquid crystal element having such properties. As a liquidcrystal element having such properties, an IPS-mode liquid crystalelement in which response time has low dependency on a gray scale can begiven. For example, by using the correction circuit 30103 shown in FIG.88E for an IPS-mode liquid crystal element in this manner, manufacturingcost can be significantly reduced and an overdriving circuit which canprevent output of the inappropriate output image signal 30104 can beobtained.

Operations which are similar to those of the circuit shown in FIGS. 88Ato 88E may be realized by software processing. As for the memory usedfor the delay circuit, another memory included in the liquid crystaldisplay device, a memory included in a device which transfers an imagedisplayed on the liquid crystal display device (e.g., a video card orthe like included in a personal computer or a device similar to thepersonal computer) can be used. Thus, intensity of overdriving,availability, or the like can be selected in accordance with user'spreference in addition to reduction in manufacturing cost.

Driving which controls a potential of a common line is described withreference to FIGS. 89A and 89B. FIG. 89A is a diagram showing aplurality of pixel circuits in which one common line is provided withrespect to one scan line in a display device using a display elementwhich has capacitive properties like a liquid crystal element. Each ofthe pixel circuits shown in FIG. 89A includes a transistor 30201, anauxiliary capacitor 30202, a display element 30203, a video signal line30204, a scan line 30205, and a common line 30206.

A gate electrode of the transistor 30201 is electrically connected tothe scan line 30205; one of a source electrode and a drain electrode ofthe transistor 30201 is electrically connected to the video signal line30204; and the other of the source electrode and the drain electrode ofthe transistor 30201 is electrically connected to one of electrodes ofthe auxiliary capacitor 30202 and one of electrodes of the displayelement 30203. In addition, the other of the electrodes of the auxiliarycapacitor 30202 is electrically connected to the common line 30206.

First, in each of pixels selected by the scan line 30205, voltagecorresponding to an image signal is applied to the display element 30203and the auxiliary capacitor 30202 through the video signal line 30204because the transistor 30201 is turned on. At this time, when the imagesignal is a signal which makes all of pixels connected to the commonline 30206 display a minimum gray scale or when the image signal is asignal which makes all of the pixels connected to the common line 30206display a maximum gray scale, it is not necessary that the image signalbe written in each of the pixels through the video signal line 30204.Voltage applied to the display element 30203 can be changed by changinga potential of the common line 30206 instead of writing the image signalthrough the video signal line 30204.

Next, FIG. 89B is a diagram showing a plurality of pixel circuits inwhich two common lines are provided with respect to one scan line in adisplay device using a display element which has capacitive propertieslike a liquid crystal element. Each of the pixel circuits shown in FIG.89B includes a transistor 30211, an auxiliary capacitor 30212, a displayelement 30213, a video signal line 30214, a scan line 30215, a firstcommon line 30216, and a second common line 30217.

A gate electrode of the transistor 30211 is electrically connected tothe scan line 30215; one of a source electrode and a drain electrode ofthe transistor 30211 is electrically connected to the video signal line30214; and the other of the source electrode and the drain electrode ofthe transistor 30211 is electrically connected to one of electrodes ofthe auxiliary capacitor 30212 and one of electrodes of the displayelement 30213. In addition, the other of the electrodes of the auxiliarycapacitor 30212 is electrically connected to the first common line30216. Further, in a pixel which is adjacent to the pixel, the other ofthe electrodes of the auxiliary capacitor 30212 is electricallyconnected to the second common line 30217.

In the pixel circuits shown in FIG. 89B, the number of pixels which areelectrically connected to one common line is small. Therefore, bychanging a potential of the first common line 30216 or the second commonline 30217 instead of writing an image signal through the video signalline 30214, frequency of changing voltage applied to the display element30213 is significantly increased. In addition, source inversion drivingor dot inversion driving can be performed. By performing sourceinversion driving or dot inversion driving, reliability of the elementcan be improved and a flicker can be suppressed.

A scanning backlight is described with reference to FIGS. 90A to 90C.FIG. 90A is a view showing a scanning backlight in which cold cathodefluorescent lamps are arranged. The scanning backlight shown in FIG. 90Aincludes a diffusion plate 30301 and N pieces of cold cathodefluorescent lamps 30302-1 to 30302-N. The N pieces of the cold cathodefluorescent lamps 30302-1 to 30302-N are arranged on the back side ofthe diffusion plate 30301, so that the N pieces of the cold cathodefluorescent lamps 30302-1 to 30302-N can be scanned while luminancethereof is changed.

Change in luminance of each of the cold cathode fluorescent lamps inscanning is described with reference to FIG. 90C. First, luminance ofthe cold cathode fluorescent lamp 30302-1 is changed for a certainperiod. After that, luminance of the cold cathode fluorescent lamp30302-2 which is provided adjacent to the cold cathode fluorescent lamp30302-1 is changed for the same period. In this manner, luminance ischanged sequentially from the cold cathode fluorescent lamp 30302-1 tothe cold cathode fluorescent lamp 30302-N. Although luminance which ischanged for a certain period is set to be lower than original luminancein FIG. 90C, it may also be higher than original luminance. In addition,although scanning is performed from the cold cathode fluorescent lamps30302-1 to 30302-N, scanning may also be performed from the cold cathodefluorescent lamps 30302-N to 30302-1, which is in a reversed order.

By performing driving as in FIGS. 90A to 90C, average luminance of thebacklight can be decreased. Therefore, power consumption of thebacklight, which mainly takes up power consumption of the liquid crystaldisplay device, can be reduced.

Note that an LED may be used as a light source of the scanningbacklight. The scanning backlight in that case is as shown in FIG. 90B.The scanning backlight shown in FIG. 90B includes a diffusion plate30311 and light sources 30312-1 to 30312-N, in each of which LEDs arearranged. When the LED is used as the light source of the scanningbacklight, there is an advantage in that the backlight can be thin andlightweight. In addition, there is also an advantage that a colorreproduction area can be widened. Further, since the LEDs which arearranged in each of the light sources 30312-1 to 30312-N can besimilarly scanned, a dot scanning backlight can also be obtained. Byusing the dot scanning backlight, image quality of a moving image can befurther improved.

Note that when the LED is used as the light source of the backlight,driving can be performed by changing luminance as shown in FIG. 90C.

Next, high frequency driving is described with reference to FIGS. 91Aand 91B. FIG. 91A is a view in which one image and one intermediateimage are displayed in one frame period 30600. A reference numeral 30601denotes an image of the frame; a reference numeral 30602 denotes anintermediate image of the frame; a reference numeral 30603 denotes animage of the next frame; and a reference numeral 30604 denotes anintermediate image of the next frame.

Note that the intermediate image 30602 of the frame may be an imagewhich is made based on an image signal of the frame and an image signalof the next frame. Alternatively, the intermediate image 30602 of theframe may be an image which is made from the image 30601 of the frame.Further alternatively, the intermediate image 30602 of the frame may bea black image. Thus, image quality of a moving image of a hold-typedisplay device can be improved. In the case where one image and oneintermediate image are displayed in the one frame period 30600, there isan advantage in that consistency with a frame rate of the image signalcan be easily obtained and an image processing circuit does not becomecomplicated.

FIG. 91B is a view in which one image and two intermediate images aredisplayed in a period having two successive one frame periods 30600(i.e., two frame periods). A reference numeral 30611 denotes an image ofthe frame; a reference numeral 30612 denotes an intermediate image ofthe frame; a reference numeral 30613 denotes an intermediate image ofthe next frame; and a reference numeral 30614 denotes an image of aframe after next.

Note that each of the intermediate image 30612 of the frame and theintermediate image 30613 of the next frame may be an image which is madebased on an image signal of the frame, an image signal of the nextframe, and an image signal of the frame after next. Alternatively, eachof the intermediate image 30612 of the frame and the intermediate image30613 of the next frame may be a black image. In the case where oneimage and two intermediate images are displayed in the two frameperiods, there is an advantage in that operating frequency of aperipheral driver circuit is made not so high and image quality of amoving image can be effectively improved.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 11

In this embodiment mode, a peripheral portion of a liquid crystal panelis described.

FIG. 55 shows an example of a liquid crystal display device including aso-called edge-light type backlight unit 20101 and a liquid crystalpanel 20107. An edge-light type corresponds to a type in which a lightsource is provided at an end of a backlight unit and fluorescence of thelight source is emitted from the entire light-emitting surface. Theedge-light type backlight unit 20101 is thin and can save power.

The backlight unit 20101 includes a diffusion plate 20102, a light guideplate 20103, a reflection plate 20104, a lamp reflector 20105, and alight source 20106.

The light source 20106 has a function of emitting light as necessary.For example, as the light source 20106, a cold cathode fluorescent lamp,a hot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like can be used.

FIGS. 56A to 56D are views each showing a detailed structure of theedge-light type backlight unit. Note that description of a diffusionplate, a light guide plate, a reflection plate, and the like is omitted.

A backlight unit 20201 shown in FIG. 56A has a structure in which a coldcathode fluorescent lamp 20203 is used as a light source. In addition, alamp reflector 20202 is provided to efficiently reflect light from thecold cathode fluorescent lamp 20203. Such a structure is often used fora large display device because luminance from the cold cathodefluorescent lamp 20203 is high.

A backlight unit 20211 shown in FIG. 56B has a structure in whichlight-emitting diodes (LEDs) 20213 are used as light sources. Forexample, the light-emitting diodes (LEDs) 20213 which emit white lightare provided at a predetermined interval. In addition, a lamp reflector20212 is provided to efficiently reflect light from the light-emittingdiodes (LEDs) 20213.

A backlight unit 20221 shown in FIG. 56C has a structure in whichlight-emitting diodes (LEDs) 20223, light-emitting diodes (LEDs) 20224,and light-emitting diodes (LEDs) 20225 of R, and B are used as lightsources. The light-emitting diodes (LEDs) 20223, the light-emittingdiodes (LEDs) 20224, and the light-emitting diodes (LEDs) 20225 of R, Gand B are each provided at a predetermined interval. By using thelight-emitting diodes (LEDs) 20223, the light-emitting diodes (LEDs)20224, and the light-emitting diodes (LEDs) 20225 of R, G, and B, colorreproductivity can be improved. In addition, a lamp reflector 20222 isprovided to efficiently reflect light from the light-emitting diodes.

A backlight unit 20231 shown in FIG. 56D has a structure in whichlight-emitting diodes (LEDs) 20233, light-emitting diodes (LEDs) 20234,and light-emitting diodes (LEDs) 20235 of R, G, and B are used as lightsources. For example, among the light-emitting diodes (LEDs) 20233, thelight-emitting diodes (LEDs) 20234, and the light-emitting diodes (LEDs)20235 of R, G, and B, the light-emitting diodes of a color with lowemission intensity (e.g., green) are provided more than otherlight-emitting diodes. By using the light-emitting diodes (LEDs) 20233,the light-emitting diodes (LEDs) 20234, and the light-emitting diodes(LEDs) 20235 of R, G, and B, color reproductivity can be improved. Inaddition, a lamp reflector 20232 is provided to efficiently reflectlight from the light-emitting diodes.

FIG. 59 shows an example of a liquid crystal display device including aso-called direct-type backlight unit and a liquid crystal panel. Adirect type corresponds to a type in which a light source is provideddirectly under a light-emitting surface and fluorescence of the lightsource is emitted from the entire light-emitting surface. Thedirect-type backlight unit can efficiently utilize the amount of emittedlight.

A backlight unit 20500 includes a diffusion plate 20501, alight-shielding plate 20502, a lamp reflector 20503, a light source20504, and a liquid crystal panel 20505.

The light source 20504 has a function of emitting light as necessary.For example, as the light source 20504, a cold cathode fluorescent lamp,a hot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like can be used.

FIG. 57 is a view showing an example of a structure of a polarizingplate (also referred to as a polarizing film).

A polarizing film 20300 includes a protective film 20301, a substratefilm 20302, a PVA polarizing film 20303, a substrate film 20304, anadhesive layer 20305, and a mold release film 20306.

When the PVA polarizing film 20303 is sandwiched by films to be basematerials (the substrate film 20302 and the substrate film 20304) fromboth sides, reliability can be improved. Note that the PVA polarizingfilm 20303 may be sandwiched by triacetylcellulose (TAC) films with highlight-transmitting properties and high durability. Note also that eachof the substrate films and the TAC films function as protective films ofpolarizer included in the PVA polarizing film 20303.

The adhesive layer 20305 which is to be attached to a glass substrate ofthe liquid crystal panel is attached to one of the substrate films (thesubstrate film 20304). Note that the adhesive layer 20305 is formed byapplying an adhesive to one of the substrate films (the substrate film20304). The mold release film 20306 (a separate film) is provided to theadhesive layer 20305.

The protective film 20301 is provided to the other one of the substratesfilms (the substrate film 20302).

A hard coating scattering layer (an anti-glare layer) may be provided ona surface of the polarizing film 20300. Since the surface of the hardcoating scattering layer has minute unevenness formed by AG treatmentand has an anti-glare function which scatters external light, reflectionof external light in the liquid crystal panel and surface reflection canbe prevented.

Note also that a treatment in which plurality of optical thin filmlayers having different refractive indexes are layered (also referred toas anti-reflection treatment or AR treatment) may be performed on thesurface of the polarizing film 20300. The plurality of layered opticalthin film layers having different refractive indexes can reducereflectivity on the surface by an interference effect of light.

FIGS. 58A to 58C are diagrams each showing an example of a system blockof the liquid crystal display device.

In a pixel portion 20405, signal lines 20412 which are extended from asignal line driver circuit 20403 are provided. In the pixel portion20405, scan lines 20410 which are extended from a scan line drivercircuit 20404 are also provided. In addition, a plurality of pixels arearranged in matrix in cross regions of the signal lines 20412 and thescan lines 20410. Note that each of the plurality of pixels includes aswitching element. Therefore, voltage for controlling inclination ofliquid crystal molecules can be separately input to each of theplurality of pixels. A structure in which a switching element isprovided in each cross region in this manner is referred to as an activematrix type. Note also that the present invention is not limited to suchan active matrix type and a structure of a passive matrix type may beused. Since the passive matrix type does not have a switching element ineach pixel, a process is simple.

A driver circuit portion 20408 includes a control circuit 20402, thesignal line driver circuit 20403, and the scan line driver circuit20404. An image signal 20401 is input to the control circuit 20402. Thesignal line driver circuit 20403 and the scan line driver circuit 20404are controlled by the control circuit 20402 in accordance with thisimage signal 20401. Therefore, the control circuit 20402 inputs acontrol signal to each of the signal line driver circuit 20403 and thescan line driver circuit 20404. Then, in accordance with this controlsignal, the signal line driver circuit 20403 inputs a video signal toeach of the signal lines 20412 and the scan line driver circuit 20404inputs a scan signal to each of the scan lines 20410. Then, theswitching element included in the pixel is selected in accordance withthe scan signal and the video signal is input to a pixel electrode ofthe pixel.

Note that the control circuit 20402 also controls a power source 20407in accordance with the image signal 20401. The power source 20407includes a unit for supplying power to a lighting unit 20406. As thelighting unit 20406, an edge-light type backlight unit or a direct-typebacklight unit can be used. Note also that a front light may be used asthe lighting unit 20406. A front light corresponds to a plate-likelighting unit including a luminous body and a light conducting body,which is attached to the front surface side of a pixel portion andilluminates the whole area. By using such a lighting unit, the pixelportion can be uniformly illuminated at low power consumption.

As shown in FIG. 58B, the scan line driver circuit 20404 includes ashift register 20441, a level shifter 20442, and a circuit functioningas a buffer 20443. A signal such as a gate start pulse (GSP) or a gateclock signal (GCK) is input to the shift register 20441.

As shown in FIG. 58C, the signal line driver circuit 20403 includes ashift register 20431, a first latch 20432, a second latch 20433, a levelshifter 20434, and a circuit functioning as a buffer 20435. The circuitfunctioning as the buffer 20435 corresponds to a circuit which has afunction of amplifying a weak signal and includes an operationalamplifier or the like. A signal such as a start pulse (SSP) is input tothe level shifter 20434 and data (DATA) such as a video signal is inputto the first latch 20432. A latch (LAT) signal can be temporally held inthe second latch 20433 and is simultaneously input to the pixel portion20405. This is referred to as line sequential driving. Therefore, when apixel is used in which not line sequential driving but dot sequentialdriving is performed, the second latch can be omitted.

Note that in this embodiment mode, various types of liquid crystalpanels can be used as the liquid crystal panel. For example, a structurein which a liquid crystal layer is sealed between two substrates can beused as the liquid crystal panel. A transistor, a capacitor, a pixelelectrode, an alignment film, or the like is formed over one of thesubstrates. A polarizing plate, a retardation plate, or a prism sheetmay be provided on the surface opposite to a top surface of the one ofthe substrates. A color filter, a black matrix, a counter electrode, analignment film, or the like is provided on the other one of thesubstrates. Note that a polarizing plate or a retardation plate may beprovided on the surface opposite to a top surface of the other one ofthe substrates. Note also that the color filter and the black matrix maybe formed over the top surface of the one of the substrates. Note alsothat three-dimensional display can be performed by providing a slit (agrid) on the top surface side of the one of the substrates or thesurface opposite to the top surface side of the one of the substrates.

Note also that each of the polarizing plate, the retardation plate, andthe prism sheet can be provided between the two substrates.Alternatively, each of the polarizing plate, the retardation plate, andthe prism sheet can be integrated with one of the two substrates.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 12

In this embodiment mode, a pixel structure and an operation of a pixelwhich can be applied to a liquid crystal display device are described.

Note that in this embodiment mode, as an operation mode of a liquidcrystal element, a TN (Twisted Nematic) mode, an IPS(In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, an MVA(Multi-domain Vertical Alignment) mode, a PVA (Patterned VerticalAlignment) mode, an ASM (Axially Symmetric aligned Micro-cell) mode, anOCB (Optical Compensated Birefringence) mode, an FLC (FerroelectricLiquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) mode,or the like can be used.

FIG. 60A is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device.

A pixel 40100 includes a transistor 40101, a liquid crystal element40102, and a capacitor 40103. A gate of the transistor 40101 isconnected to a wiring 40105. A first electrode of the transistor 40101is connected to a wiring 40104. A second electrode of the transistor40101 is connected to a first electrode of the liquid crystal element40102 and a first electrode of the capacitor 40103. A second electrodeof the liquid crystal element 40102 corresponds to a counter electrode40107. A second electrode of the capacitor 40103 is connected to awiring 40106.

The wiring 40104 functions as a signal line. The wiring 40105 functionsas a scan line. The wiring 40106 functions as a capacitor line. Thetransistor 40101 functions as a switch. The capacitor 40103 functions asa storage capacitor.

It is only necessary that the transistor 40101 function as a switch, andthe transistor 40101 may be a P-channel transistor or an N-channeltransistor.

FIG. 60B is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device. In particular, FIG. 60Bis a diagram showing an example of a pixel structure which can beapplied to a liquid crystal display device suitable for a lateralelectric field mode (including an IPS mode and an FFS mode).

A pixel 40110 includes a transistor 40111, a liquid crystal element40112, and a capacitor 40113. A gate of the transistor 40111 isconnected to a wiring 40115. A first electrode of the transistor 40111is connected to a wiring 40114. A second electrode of the transistor40111 is connected to a first electrode of the liquid crystal element40112 and a first electrode of the capacitor 40113. A second electrodeof the liquid crystal element 40112 is connected to a wiring 40116. Asecond electrode of the capacitor 40103 is connected to the wiring40116.

The wiring 40114 functions as a signal line. The wiring 40115 functionsas a scan line. The wiring 40116 functions as a capacitor line. Thetransistor 40111 functions as a switch. The capacitor 40113 functions asa storage capacitor.

It is only necessary that the transistor 40111 function as a switch, andthe transistor 40111 may be a P-channel transistor or an N-channeltransistor.

FIG. 61 is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device. In particular, FIG. 61is a diagram showing an example of a pixel structure in which anaperture ratio of a pixel can be increased by reducing the number ofwirings.

FIG. 61 shows two pixels which are provided in the same column direction(a pixel 40200 and a pixel 40210). For example, when the pixel 40200 isprovided in an N-th row, the pixel 40210 is provided in an (N+1)th row.

The pixel 40200 includes a transistor 40201, a liquid crystal element40202, and a capacitor 40203. A gate of the transistor 40201 isconnected to a wiring 40205. A first electrode of the transistor 40201is connected to a wiring 40204. A second electrode of the transistor40201 is connected to a first electrode of the liquid crystal element40202 and a first electrode of the capacitor 40203. A second electrodeof the liquid crystal element 40202 corresponds to a counter electrode40207. A second electrode of the capacitor 40203 is connected to awiring which is the same as a wiring connected to a gate of a transistorof the previous row.

The pixel 40210 includes a transistor 40211, a liquid crystal element40212, and a capacitor 40213. A gate of the transistor 40211 isconnected to a wiring 40215. A first electrode of the transistor 40211is connected to the wiring 40204. A second electrode of the transistor40211 is connected to a first electrode of the liquid crystal element40212 and a first electrode of the capacitor 40213. A second electrodeof the liquid crystal element 40212 corresponds to a counter electrode40217. A second electrode of the capacitor 40213 is connected to awiring which is the same as the wiring connected to the gate of thetransistor of the previous row (the wiring 40205).

The wiring 40204 functions as a signal line. The wiring 40205 functionsas a scan line of the N-th row. The wiring 40205 also functions as acapacitor line of the (N+1)th row. The transistor 40201 functions as aswitch. The capacitor 40203 functions as a storage capacitor.

The wiring 40215 functions as a scan line of the (N+1)th row. The wiring40215 also functions as a capacitor line of an (N+2)th row. Thetransistor 40211 functions as a switch. The capacitor 40213 functions asa storage capacitor.

It is only necessary that each of the transistor 40201 and thetransistor 40211 function as a switch, and each of the transistor 40201and the transistor 40211 may be a P-channel transistor or an N-channeltransistor.

FIG. 62 is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device. In particular, FIG. 62is a diagram showing an example of a pixel structure in which a viewingangle can be improved by using a subpixel.

A pixel 40320 includes a subpixel 40300 and a subpixel 40310. Althoughthe case in which the pixel 40320 includes two subpixels is described,the pixel 40320 may include three or more subpixels.

The subpixel 40300 includes a transistor 40301, a liquid crystal element40302, and a capacitor 40303. A gate of the transistor 40301 isconnected to a wiring 40305. A first electrode of the transistor 40301is connected to a wiring 40304. A second electrode of the transistor40301 is connected to a first electrode of the liquid crystal element40302 and a first electrode of the capacitor 40303. A second electrodeof the liquid crystal element 40302 corresponds to a counter electrode40307. A second electrode of the capacitor 40303 is connected to awiring 40306.

The subpixel 40310 includes a transistor 40311, a liquid crystal element40312, and a capacitor 40313. A gate of the transistor 40311 isconnected to a wiring 40315. A first electrode of the transistor 40311is connected to the wiring 40304. A second electrode of the transistor40311 is connected to a first electrode of the liquid crystal element40312 and a first electrode of the capacitor 40313. A second electrodeof the liquid crystal element 40312 corresponds to a counter electrode40317. A second electrode of the capacitor 40313 is connected to awiring 40306.

The wiring 40304 functions as a signal line. The wiring 40305 functionsas a scan line. The wiring 40315 functions as a signal line. The wiring40306 functions as a capacitor line. The transistor 40301 functions as aswitch. The transistor 40311 functions as a switch. The capacitor 40303functions as a storage capacitor. The capacitor 40313 functions as astorage capacitor.

It is only necessary that the transistor 40301 function as a switch, andthe transistor 40301 may be a P-channel transistor or an N-channeltransistor. It is only necessary that the transistor 40311 function as aswitch, and the transistor 40311 may be a P-channel transistor or anN-channel transistor.

A video signal input to the subpixel 40300 may be a value which isdifferent from that of a video signal input to the subpixel 40310. Inthis case, the viewing angle can be widened because alignment of liquidcrystal molecules of the liquid crystal element 40302 and alignment ofliquid crystal molecules of the liquid crystal element 40312 can bevaried from each other.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 13

In this embodiment mode, various liquid crystal modes are described.

First, various liquid crystal modes are described with reference tocross-sectional views.

FIGS. 63A and 63B are schematic views of cross sections of a TN mode.

A liquid crystal layer 50100 is held between a first substrate 50101 anda second substrate 50102 which are provided so as to be opposite to eachother. A first electrode 50105 is formed on a top surface of the firstsubstrate 50101. A second electrode 50106 is formed on a top surface ofthe second substrate 50102. A first polarizing plate 50103 is providedon a surface of the first substrate 50101, which does not face theliquid crystal layer 50100. A second polarizing plate 50104 is providedon a surface of the second substrate 50102, which does not face theliquid crystal layer 50100. Note that the first polarizing plate 50103and the second polarizing plate 50104 are provided so as to be in across nicol state.

The first polarizing plate 50103 may be provided on the top surface ofthe first substrate 50101, i.e., may be provided between the firstsubstrate 50101 and the liquid crystal layer 50100. The secondpolarizing plate 50104 may be provided on the top surface of the secondsubstrate 50102, i.e., may be provided between the second substrate50102 and the liquid crystal layer 50100.

It is only necessary that at least one of the first electrode 50105 andthe second electrode 50106 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50105 and the second electrode50106 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 63A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50105 and the second electrode50106 (referred to as a vertical electric field mode).

FIG. 63B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50105 and the secondelectrode 50106.

FIGS. 64A and 64B are schematic views of cross sections of a VA mode. Inthe VA mode, liquid crystal molecules are aligned such that they arevertical to a substrate when there is no electric field.

A liquid crystal layer 50200 is held between a first substrate 50201 anda second substrate 50202 which are provided so as to be opposite to eachother. A first electrode 50205 is formed on a top surface of the firstsubstrate 50201. A second electrode 50206 is formed on a top surface ofthe second substrate 50202. A first polarizing plate 50203 is providedon a surface of the first substrate 50201, which does not face theliquid crystal layer 50200. A second polarizing plate 50204 is providedon a surface of the second substrate 50202, which does not face theliquid crystal layer 50200. Note that the first polarizing plate 50203and the second polarizing plate 50204 are provided so as to be in across nicol state.

The first polarizing plate 50203 may be provided on the top surface ofthe first substrate 50201, i.e., may be provided between the firstsubstrate 50201 and the liquid crystal layer 50200. The secondpolarizing plate 50204 may be provided on the top surface of the secondsubstrate 50202, i.e., may be provided between the second substrate50202 and the liquid crystal layer 50200.

It is only necessary that at least one of the first electrode 50205 andthe second electrode 50206 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50205 and the second electrode50206 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 64A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50205 and the second electrode50206 (referred to as a vertical electric field mode).

FIG. 64B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50205 and the secondelectrode 50206.

FIGS. 64C and 64D are schematic views of cross sections of an MVA mode.In the MVA mode, viewing angle dependency of each portion is compensatedby each other.

A liquid crystal layer 50210 is held between a first substrate 50211 anda second substrate 50212 which are provided so as to be opposite to eachother. A first electrode 50215 is formed on a top surface of the firstsubstrate 50211. A second electrode 50216 is formed on a top surface ofthe second substrate 50212. A first projection 50217 for controllingalignment is formed on the first electrode 50215. A second projection50218 for controlling alignment is formed over the second electrode50216. A first polarizing plate 50213 is provided on a surface of thefirst substrate 50211, which does not face the liquid crystal layer50210. A second polarizing plate 50214 is provided on a surface of thesecond substrate 50212, which does not face the liquid crystal layer50210. Note that the first polarizing plate 50213 and the secondpolarizing plate 50214 are provided so as to be in a cross nicol state.

The first polarizing plate 50213 may be provided on the top surface ofthe first substrate 50211, i.e., may be provided between the firstsubstrate 50211 and the liquid crystal layer 50210. The secondpolarizing plate 50214 may be provided on the top surface of the secondsubstrate 50212, i.e., may be provided between the second substrate50212 and the liquid crystal layer 50210.

It is only necessary that at least one of the first electrode 50215 andthe second electrode 50216 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50215 and the second electrode50216 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 64C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50215 and the second electrode50216 (referred to as a vertical electric field mode).

FIG. 64D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50215 and the secondelectrode 50216.

FIGS. 65A and 65B are schematic views of cross sections of an OCB mode.In the OCB mode, viewing angle dependency is low because alignment ofliquid crystal molecules in a liquid crystal layer can be opticallycompensated. This state of the liquid crystal molecules is referred toas bend alignment.

A liquid crystal layer 50300 is held between a first substrate 50301 anda second substrate 50302 which are provided so as to be opposite to eachother. A first electrode 50305 is formed on a top surface of the firstsubstrate 50301. A second electrode 50306 is formed on a top surface ofthe second substrate 50302. A first polarizing plate 50303 is providedon a surface of the first substrate 50301, which does not face theliquid crystal layer 50300. A second polarizing plate 50304 is providedon a surface of the second substrate 50302, which does not face theliquid crystal layer 50300. Note that the first polarizing plate 50303and the second polarizing plate 50304 are provided so as to be in across nicol state.

The first polarizing plate 50303 may be provided on the top surface ofthe first substrate 50301, i.e., may be provided between the firstsubstrate 50301 and the liquid crystal layer 50300. The secondpolarizing plate 50304 may be provided on the top surface of the secondsubstrate 50302, i.e., may be provided between the second substrate50302 and the liquid crystal layer 50300.

It is only necessary that at least one of the first electrode 50305 andthe second electrode 50306 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50305 and the second electrode50306 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 65A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50305 and the second electrode50306 (referred to as a vertical electric field mode).

FIG. 65B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50305 and the secondelectrode 50306.

FIGS. 65C and 65D are schematic views of cross sections of an FLC modeor an AFLC mode.

A liquid crystal layer 50310 is held between a first substrate 50311 anda second substrate 50312 which are provided so as to be opposite to eachother. A first electrode 50315 is formed on a top surface of the firstsubstrate 50311. A second electrode 50316 is formed on a top surface ofthe second substrate 50312. A first polarizing plate 50313 is providedon a surface of the first substrate 50311, which does not face theliquid crystal layer 50310. A second polarizing plate 50314 is providedon a surface of the second substrate 50312, which does not face theliquid crystal layer 50310. Note that the first polarizing plate 50313and the second polarizing plate 50314 are provided so as to be in across nicol state.

The first polarizing plate 50313 may be provided on the top surface ofthe first substrate 50311, i.e., may be provided between the firstsubstrate 50311 and the liquid crystal layer 50310. The secondpolarizing plate 50314 may be provided on the top surface of the secondsubstrate 50312, i.e., may be provided between the second substrate50312 and the liquid crystal layer 50310.

It is only necessary that at least one of the first electrode 50315 andthe second electrode 50316 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50315 and the second electrode50316 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 65C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50315 and the second electrode50316 (referred to as a vertical electric field mode).

FIG. 65D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50315 and the secondelectrode 50316.

FIGS. 66A and 66B are schematic views of cross sections of an IPS mode.In the IPS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate side is used.

A liquid crystal layer 50400 is held between a first substrate 50401 anda second substrate 50402 which are provided so as to be opposite to eachother. A first electrode 50405 and a second electrode 50406 are formedon a top surface of the second substrate 50402. A first polarizing plate50403 is provided on a surface of the first substrate 50401, which doesnot face the liquid crystal layer 50400. A second polarizing plate 50404is provided on a surface of the second substrate 50402, which does notface the liquid crystal layer 50400. Note that the first polarizingplate 50403 and the second polarizing plate 50404 are provided so as tobe in a cross nicol state.

The first polarizing plate 50403 may be provided on the top surface ofthe first substrate 50401, i.e., may be provided between the firstsubstrate 50401 and the liquid crystal layer 50400. The secondpolarizing plate 50404 may be provided on the top surface of the secondsubstrate 50402, i.e., may be provided between the second substrate50402 and the liquid crystal layer 50400.

It is only necessary that at least one of the first electrode 50405 andthe second electrode 50406 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50405 and the second electrode50406 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 66A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50405 and the second electrode50406 (referred to as a vertical electric field mode).

FIG. 66B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50405 and the secondelectrode 50406.

FIGS. 66C and 66D are schematic views of cross sections of an FFS mode.In the FFS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate side is used.

A liquid crystal layer 50410 is held between a first substrate 50411 anda second substrate 50412 which are provided so as to be opposite to eachother. A second electrode 50416 is formed on a top surface of the secondsubstrate 50412. An insulating film 50417 is formed on a top surface ofthe second electrode 50416. A first electrode 50415 is formed over theinsulating film 50417. A first polarizing plate 50413 is provided on asurface of the first substrate 50411, which does not face the liquidcrystal layer 50410. A second polarizing plate 50414 is provided on asurface of the second substrate 50412, which does not face the liquidcrystal layer 50410. Note that the first polarizing plate 50413 and thesecond polarizing plate 50414 are provided so as to be in a cross nicolstate.

The first polarizing plate 50413 may be provided on the top surface ofthe first substrate 50411, i.e., may be provided between the firstsubstrate 50411 and the liquid crystal layer 50410. The secondpolarizing plate 50414 may be provided on the top surface of the secondsubstrate 50412, i.e., may be provided between the second substrate50412 and the liquid crystal layer 50410.

It is only necessary that at least one of the first electrode 50415 andthe second electrode 50416 have light-transmitting properties (atransmissive or reflective liquid crystal display device).Alternatively, both the first electrode 50415 and the second electrode50416 may have light-transmitting properties, and part of one of theelectrodes may have reflectivity (a semi-transmissive liquid crystaldisplay device).

FIG. 66C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50415 and the second electrode50416 (referred to as a vertical electric field mode).

FIG. 66D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50415 and the secondelectrode 50416.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 14

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a liquid crystal displaydevice is described.

A pixel structure in the case where each liquid crystal mode and atransistor are combined is described with reference to cross-sectionalviews of a pixel.

Note that as the transistor, a thin film transistor (a TFT) including anon-single crystalline semiconductor layer typified by amorphoussilicon, polycrystalline silicon, micro crystalline (also referred to assemi-amorphous) silicon, or the like can be used.

As a structure of the transistor, a top-gate structure, a bottom-gatestructure, or the like can be used. Note that a channel-etchedtransistor, a channel-protective transistor, or the like can be used asa bottom-gate transistor.

FIG. 67 is an example of a cross-sectional view of a pixel in the casewhere a TN mode and a transistor are combined. A liquid crystal 10111having liquid crystal molecules 10118 is held between a first substrate10101 and a second substrate 10116. A transistor, a pixel electrode, analignment film, and the like are provided over the first substrate10101, and a light-shielding film 10114, a color filter 10115, a counterelectrode, an alignment film, and the like are provided on the secondsubstrate 10116. In addition, a spacer 10117 is provided between thefirst substrate 10101 and the second substrate 10116. By applying thepixel structure shown in FIG. 67 to a liquid crystal display device, aliquid crystal display device can be formed at low cost.

FIG. 68A is an example of a cross-sectional view of a pixel in the casewhere an MVA (Multi-domain Vertical Alignment) mode and a transistor arecombined. A liquid crystal 10211 having liquid crystal molecules 10218is held between a first substrate 10201 and a second substrate 10216. Atransistor, a pixel electrode, an alignment film, and the like areprovided over the first substrate 10201, and a light-shielding film10214, a color filter 10215, a counter electrode, an alignment controlprojection 10219, an alignment film, and the like are provided on thesecond substrate 10216. In addition, a spacer 10217 is provided betweenthe first substrate 10201 and the second substrate 10216. By applyingthe pixel structure shown in FIG. 68A to a liquid crystal displaydevice, a liquid crystal display device having a wide viewing angle,high response speed, and high contrast can be obtained.

FIG. 68B is an example of a cross-sectional view of a pixel in the casewhere a PVA (Patterned Vertical Alignment) mode and a transistor arecombined. A liquid crystal 10241 having liquid crystal molecules 10248is held between a first substrate 10231 and a second substrate 10246. Atransistor, a pixel electrode, an alignment film, and the like areprovided over the first substrate 10231, and a light-shielding film10244, a color filter 10245, a counter electrode, an alignment film, andthe like are provided on the second substrate 10246. Note that the pixelelectrode includes an electrode notch portion 10249. In addition, aspacer 10247 is provided between the first substrate 10231 and thesecond substrate 10246. By applying the pixel structure shown in FIG.68B to a liquid crystal display device, a liquid crystal display devicehaving a wide viewing angle, high response speed, and high contrast canbe obtained.

FIG. 69A is an example of a cross-sectional view of a pixel in the casewhere an IPS (In-Plane-Switching) mode and a transistor are combined. Aliquid crystal 10311 having liquid crystal molecules 10318 is heldbetween a first substrate 10301 and a second substrate 10316. Atransistor, a pixel electrode, a common electrode, an alignment film,and the like are provided over the first substrate 10301, and alight-shielding film 10314, a color filter 10315, an alignment film, andthe like are provided on the second substrate 10316. In addition, aspacer 10317 is provided between the first substrate 10301 and thesecond substrate 10316. By applying the pixel structure shown in FIG.69A to a liquid crystal display device, a liquid crystal display devicetheoretically having a wide viewing angle and response speed which haslow dependency on a gray scale can be obtained.

FIG. 69B is an example of a cross-sectional view of a pixel in the casewhere an FFS (Fringe Field Switching) mode and a transistor arecombined. A liquid crystal 10341 having liquid crystal molecules 10348is held between a first substrate 10331 and a second substrate 10346. Atransistor, a pixel electrode, a common electrode, an alignment film,and the like are provided over the first substrate 10331, and alight-shielding film 10344, a color filter 10345, an alignment film, andthe like are provided on the second substrate 10346. In addition, aspacer 10347 is provided between the first substrate 10331 and thesecond substrate 10346. By applying the pixel structure shown in FIG.69B to a liquid crystal display device, a liquid crystal display devicetheoretically having a wide viewing angle and response speed which haslow dependency on a gray scale can be obtained.

Here, materials which can be used for conductive layers or insulatingfilms are described.

As a first insulating film 10102 in FIG. 67, a first insulating film10202 in FIG. 68A, a first insulating film 10232 in FIG. 68B, a firstinsulating film 10302 in FIG. 69A, or a first insulating film 10332 inFIG. 69B, an insulating film such as a silicon oxide film, a siliconnitride film, or a silicon oxynitride (SiO_(x)N_(y)) film can be used.Alternatively, an insulating film having a stacked-layer structure inwhich two or more of a silicon oxide film, a silicon nitride film, asilicon oxynitride (SiO_(x)N_(y)) film, and the like are combined can beused.

As a first conductive layer 10103 in FIG. 67, a first conductive layer10203 in FIG. 68A, a first conductive layer 10233 in FIG. 68B, a firstconductive layer 10303 in FIG. 69A, or a first conductive layer 10333 inFIG. 69B, Mo, Ti, Al, Nd, Cr, or the like can be used. Alternatively, astacked-layer structure in which two or more of Mo, Ti, Al, Nd, Cr, andthe like are combined can be used.

As a second insulating film 10104 in FIG. 67, a second insulating film10204 in FIG. 68A, a second insulating film 10234 in FIG. 68B, a secondinsulating film 10304 in FIG. 69A, or a second insulating film 10334 inFIG. 69B, a thermal oxide film, a silicon oxide film, a silicon nitridefilm, a silicon oxynitride film, or the like can be used. Alternatively,a stacked-Layer structure in which two or more of a thermal oxide film,a silicon oxide film, a silicon nitride film, a silicon oxynitride film,and the like are combined can be used. Note that a silicon oxide film ispreferable in a portion which is in contact with a semiconductor layer.This is because a trap level at an interface with the semiconductorlayer is decreased when a silicon oxide film is used. Note also that asilicon nitride film is preferable in a portion which is in contact withMo. This is because a silicon nitride film does not oxidize Mo.

As a first semiconductor layer 10105 in FIG. 67, a first semiconductorlayer 10205 in FIG. 68A, a first semiconductor layer 10235 in FIG. 68B,a first semiconductor layer 10305 in FIG. 69A, or a first semiconductorlayer 10335 in FIG. 69B, silicon, silicon germanium (SiGe), or the likecan be used.

As a second semiconductor layer 10106 in FIG. 67, a second semiconductorlayer 10206 in FIG. 68A, a second semiconductor layer 10236 in FIG. 68B,a second semiconductor layer 10306 in FIG. 69A, or a secondsemiconductor layer 10336 in FIG. 69B, silicon or the like includingphosphorus can be used, for example.

As a light-transmitting material of a second conductive layer 10107, athird conductive layer 10109, and a fourth conductive layer 10113 inFIG. 67; a second conductive layer 10207, a third conductive layer10209, and a fourth conductive layer 10213 in FIG. 68A; a secondconductive layer 10237, a third conductive layer 10239, and a fourthconductive layer 10243 in FIG. 68B; a second conductive layer 10307 anda third conductive layer 10309 in FIG. 69A; or a second conductive layer10337, a third conductive layer 10339, and a fourth conductive layer10343 in FIG. 69B, an indium tin oxide (ITO) film formed by mixing tinoxide into indium oxide, an indium tin silicon oxide (ITSO) film formedby mixing silicon oxide into indium tin oxide (ITO), an indium zincoxide (IZO) film formed by mixing zinc oxide into indium oxide, a zincoxide film, a tin oxide film, or the like can be used. Note that IZO isa light-transmitting conductive material formed by sputtering using atarget in which 2 to 20 wt % of zinc oxide (ZnO) is mixed into ITO.

As a reflective material of the second conductive layer 10107 and thethird conductive layer 10109 in FIG. 67; the second conductive layer10207 and the third conductive layer 10209 in FIG. 68A; the secondconductive layer 10237 and the third conductive layer 10239 in FIG. 68B;the second conductive layer 10307 and the third conductive layer 10309in FIG. 69A; or the second conductive layer 10337, the third conductivelayer 10339, and the fourth conductive layer 10343 in FIG. 69B, Ti, Mo,Ta, Cr, W, Al, or the like can be used. Alternatively, a two-layerstructure in which Al and Ti, Mo, Ta, Cr, or W are stacked, or athree-layer structure in which Al is interposed between metals such asTi, Mo, Ta, Cr, and W may be used.

As the third insulating film 10108 in FIG. 67, the third insulating film10208 in FIG. 68A, the third insulating film 10238 in FIG. 68B, thethird conductive layer 10239 in FIG. 68B, the third insulating film10308 in FIG. 69A, or the third insulating film 10338 and the fourthinsulating film 10349 in FIG. 69B, an inorganic material (e.g., siliconoxide, silicon nitride, or silicon oxynitride), an organic compoundmaterial having a low dielectric constant (e.g., a photosensitive ornonphotosensitive organic resin material), or the like can be used.Alternatively, a material including siloxane can be used. Note thatsiloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substituent, an organic groupincluding at least hydrogen (e.g., an alkyl group or aromatichydrocarbon) is used. Alternatively, a fluoro group may be used as thesubstituent. Further alternatively, the organic group including at leasthydrogen and the fluoro group may be used as the substituent.

As a first alignment film 10110 and a second alignment film 10112 inFIG. 67; a first alignment film 10210 and a second alignment film 10212in FIG. 68A; a first alignment film 10240 and a second alignment film10242 in FIG. 68B; a first alignment film 10310 and a second alignmentfilm 10312 in FIG. 69A; or a first alignment film 10340 and a secondalignment film 10342 in FIG. 69B, a film of a high molecular compoundsuch as polyimide can be used.

Next, the pixel structure in the case where each liquid crystal mode andthe transistor are combined is described with reference to a top planview (a layout diagram) of the pixel.

Note that as the liquid crystal mode, a TN (Twisted Nematic) mode, anIPS (In-Plane-Switching) mode, an FFS (Fringe Field Switching) mode, anMVA (Multi-domain Vertical Alignment) mode, a PVA (Patterned VerticalAlignment) mode, an ASM (Axially Symmetric aligned Micro-cell) mode, anOCB (Optical Compensated Birefringence) mode, an FLC (FerroelectricLiquid Crystal) mode, an AFLC (AntiFerroelectric Liquid Crystal) mode,or the like can be used.

FIG. 70 is an example of a top plan view of a pixel in the case where aTN mode and a transistor are combined. By applying the pixel structureshown in FIG. 70 to a liquid crystal display device, a liquid crystaldisplay device can be formed at low cost.

The pixel shown in FIG. 70 includes a scan line 10401, a video signalline 10402, a capacitor line 10403, a transistor 10404, a pixelelectrode 10405, and a pixel capacitor 10406.

FIG. 71A is an example of a top plan view of a pixel in the case wherean MVA mode and a transistor are combined. By applying the pixelstructure shown in FIG. 71A to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle, high response speed,and high contrast can be obtained.

The pixel shown in FIG. 71A includes a scan line 10501, a video signalline 10502, a capacitor line 10503, a transistor 10504, a pixelelectrode 10505, a pixel capacitor 10506, and an alignment controlprojection 10507.

FIG. 71B is an example of a top plan view of a pixel in the case where aPVA mode and a transistor are combined. By applying the pixel structureshown in FIG. 71B to a liquid crystal display device, a liquid crystaldisplay device having a wide viewing angle, high response speed, andhigh contrast can be obtained.

The pixel shown in FIG. 71B includes a scan line 10511, a video signalline 10512, a capacitor line 10513, a transistor 10514, a pixelelectrode 10515, a pixel capacitor 10516, and an electrode notch portion10517.

FIG. 72A is an example of a top plan view of a pixel in the case wherean IPS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 72A to a liquid crystal display device, a liquidcrystal display device theoretically having a wide viewing angle andresponse speed which has low dependency on a gray scale can be obtained.

The pixel shown in FIG. 72A includes a scan line 10601, a video signalline 10602, a common electrode 10603, a transistor 10604, and a pixelelectrode 10605.

FIG. 72B is an example of a top plan view of a pixel in the case wherean FFS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 72B to a liquid crystal display device, a liquidcrystal display device theoretically having a wide viewing angle andresponse speed which has low dependency on a gray scale can be obtained.

The pixel shown in FIG. 72B includes a scan line 10611, a video signalline 10612, a common electrode 10613, a transistor 10614, and a pixelelectrode 10615.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 15

In this embodiment mode, a structure and an operation of a pixel in adisplay device are described.

FIGS. 73A and 73B are timing charts showing an example of digital timeratio gray scale driving. The timing chart in FIG. 73A shows a drivingmethod in which a signal writing period (an address period) to a pixeland a light-emitting period (a sustain period) are divided.

One frame period is a period for fully displaying an image for onedisplay region. One frame period includes a plurality of subframeperiods, and one subframe period includes an address period and asustain period. Address periods Ta1 to Ta4 indicate time for writingsignals to pixels in all rows, and periods Tb1 to Tb4 indicate time forwriting signals to pixels in one row (or one pixel). Sustain periods Ts1to Ts4 indicate time for maintaining a lighting state or a non-lightingstate in accordance with a video signal written to the pixel, and aratio of the length of the sustain periods is set to satisfyTs1:Ts2:Ts3:Ts4=2³:2²:2¹:2⁰=8:4:2:1. A gray scale is expressed dependingon which sustain period light emission is performed.

Here, a pixel of the i-th row is described with reference to FIG. 73B.First, in the address period Ta1, a pixel selection signal is input to ascan line in order from a first row, and in a period Tb1(i) in theaddress period Ta1, the pixel of the i-th row is selected. Then, whilethe pixel of the i-th row is selected, a video signal is input to thepixel of the i-th row from a signal line. Then, when the video signal iswritten to the pixel of the i-th row, the pixel of the i-th rowmaintains the signal until a signal is input again. Lighting andnon-lighting of the pixel of the i-th row in the sustain period Ts1 arecontrolled by the written video signal. Similarly, in the addressperiods Ta2, Ta3, and Ta4, a video signal is input to the pixel of thei-th row, and lighting and non-lighting of the pixel of the i-th row inthe sustain periods Ts2, Ts3, and Ts4 are controlled by the videosignal. Then, in each subframe period, a pixel to which a signal for notlighting in the address period and for lighting when the sustain periodstarts after the address period ends is written is lit.

Here, the case where a 4-bit gray scale is expressed; however, thenumber of bits and the number of gray scales are not limited to these.Note that lighting is not needed to be performed in order of Ts1, Ts2,Ts3, and Ts4, and the order may be random or light emission may beperformed in the period divided into a plurality of periods. A ratio oflighting times of Ts1, Ts2, Ts3, and Ts4 is not needed to bepower-of-two, and may be the same length or slightly different from apower of two.

Next, a driving method when a signal writing period (an address period)to a pixel and a light-emitting period (a sustain period) are notdivided is described. A pixel in a row in which a writing operation of avideo signal is completed maintains the signal until another signal iswritten to the pixel (or the signal is erased). Data holding time is aperiod between the writing operation and until another signal is writtento the pixel. In the data holding time, the pixel is lit or not lit inaccordance with the video signal written to the pixel. The sameoperations are performed until the last row, and the address periodends. Then, an operation proceeds to a signal writing operation in anext subframe period sequentially from a row in which the data holdingtime ends.

As described above, in the case of a driving method in which a pixel islit or not lit in accordance with a video signal written to the pixelimmediately after the signal writing operation is completed and the dataholding time starts, signals cannot be input to two rows at the sametime. Accordingly, address periods need to be prevented fromoverlapping. Therefore, the data holding time cannot be made shorterthan the address period. As a result, it becomes difficult to performhigh-level gray scale display.

Thus, the data holding time is set to be shorter than the address periodby providing an erasing period. FIG. 74A shows a driving method when thedata holding time is set shorter than the address period by providing anerasing period.

Here, the pixel of the i-th row is described with reference to FIG. 74B.In the address period Ta1, a pixel scan signal is input to a scan linein order from a first row, and a pixel is selected. Then, in the periodTb1(i), while the pixel of the i-th row is selected, a video signal isinput to the pixel of the i-th row. Then, when the video signal iswritten to the pixel of the i-th row, the pixel of the i-th rowmaintains the signal until a signal is input again. Lighting andnon-lighting of the pixel of the i-th row in the sustain period Ts1(i)are controlled by the written video signal. That is, the pixel of thei-th row is lit or not lit in accordance with the video signal writtento the pixel immediately after the writing operation of the video signalto the i-th row is completed. Similarly, in the address periods Ta2,Ta3, and Ta4, a video signal is input to the pixel of the i-th row, andlighting and non-lighting of the pixel of the i-th row in the sustainperiods Ts2, Ts3, and Ts4 are controlled by the video signal. Then, theend of a sustain period Ts4(i) is set by the start of an erasingoperation. This is because the pixel is forced to be not lit regardlessof the video signal written to the pixel of the i-th row in an erasingtime Te(i). That is, the data holding time of the pixel of the i-th rowends when the erasing time Te(i) starts.

Thus, a display device with a high-level gray scale, a high duty ratio(a ratio of a lighting period in one frame period) can be provided, inwhich data holding time is shorter than an address period withoutdividing the address period and a sustain period can be provided.Reliability of a display element can be improved because instantaneousluminance can be lowered.

Here, the case where a 4-bit gray scale is expressed; however, thenumber of bits and the number of gray scales are not limited to these.Note that lighting is not needed to be performed in order of Ts1, Ts2,Ts3, and Ts4, and the order may be random or light emission may beperformed in the period divided into a plurality of periods. A ratio oflighting times of Ts1, Ts2, Ts3, and Ts4 is not needed to bepower-of-two, and may be the same length or slightly different from apower of two.

A structure and an operation of a pixel to which digital time ratio grayscale driving can be applied are described.

FIG. 75 is a diagram showing an example of a pixel structure to whichdigital time ratio gray scale driving can be applied.

A pixel 80300 includes a switching transistor 80301, a drivingtransistor 80302, a light-emitting element 80304, and a capacitor 80303.A gate of the switching transistor 80301 is connected to a scan line80306; a first electrode (one of a source electrode and a drainelectrode) of the switching transistor 80301 is connected to a signalline 80305; and a second electrode (the other of the source electrodeand the drain electrode) of the switching transistor 80301 is connectedto a gate of the driving transistor 80302. The gate of the drivingtransistor 80302 is connected to a power supply line 80307 through thecapacitor 80303; a first electrode of the driving transistor 80302 isconnected to the power supply line 80307; and a second electrode of thedriving transistor 80302 is connected to a first electrode (a pixelelectrode) of the light-emitting element 80304. A second electrode ofthe light-emitting element 80304 corresponds to a common electrode80308.

The second electrode of the light-emitting element 80304 (the commonelectrode 80308) is set to a low power supply potential. The low powersupply potential is a potential satisfying the low power supplypotential<a high power supply potential based on the high power supplypotential set to the power supply line 80307. As the low power supplypotential, GND, 0 V, and the like may be employed, for example. Apotential difference between the high power supply potential and the lowpower supply potential is applied to the light-emitting element 80304,and current is supplied to the light-emitting element 80304. Here, inorder to make the light-emitting element 80304 emit light, eachpotential is set so that the potential difference between the high powersupply potential and the low power supply potential is a forwardthreshold voltage or more.

Gate capacitance of the driving transistor 80302 may be used as asubstitute for the capacitor 80303, so that the capacitor 80303 can beomitted. The gate capacitance of the driving transistor 80302 may beformed in a region where a source region, a drain region, an LDD region,overlaps with the gate electrode. Alternatively, capacitance may beformed between a channel region and the gate electrode.

In the case of voltage-input voltage driving method, a video signal isinput to the gate of the driving transistor 80302 so that the drivingtransistor 80302 is in either of two states of being sufficiently turnedon and turned off. That is, the driving transistor 80302 operates in alinear region.

The video signal such that the driving transistor 80302 operates in asaturation region is input, so that current can be supplied to thelight-emitting element 80304. When the light-emitting element 80304 isan element luminance of which is determined in accordance with current,luminance decay due to deterioration of the light-emitting element 80304can be suppressed. Further, when the video signal is an analog signal,current corresponding to the video signal can be supplied to thelight-emitting element 80304. In this case, analog gray scale drive canbe performed.

A structure and an operation of a pixel called a threshold voltagecompensation pixel are described. A threshold voltage compensation pixelcan be applied to digital time gray scale drive and analog gray scaledrive.

FIG. 76 is a diagram showing an example of a structure of a pixel calleda threshold voltage compensation pixel.

The pixel in FIG. 76 includes a driving transistor 80600, a first switch80601, a second switch 80602, a third switch 80603, a first capacitor80604, a second capacitor 80605, and a light-emitting element 80620. Agate of the driving transistor 80600 is connected to a signal line 80611through the first capacitor 80604 and the first switch 80601 in thisorder. Further, the gate of the driving transistor 80600 is connected toa power supply line 80612 through the second capacitor 80605. A firstelectrode of the driving transistor 80600 is connected to the powersupply line 80612. A second electrode of the driving transistor 80600 isconnected to a first electrode of the light-emitting element 80620through the third switch 80603. Further, the second electrode of thedriving transistor 80600 is connected to the gate of the drivingtransistor 80600 through the first electrode of the light-emittingelement 80620. A second electrode of the light-emitting element 80620corresponds to a common electrode 80621. Note that on/off of the firstswitch 80601, the second switch 80602, and the third switch 80603 iscontrolled by a signal input to a first scan line 80613, a signal inputto a second scan line 80615, and a signal input to a third scan line80614, respectively.

A pixel structure shown in FIG. 76 is not limited this. For example, aswitch, a resistor, a capacitor, a transistor, a logic circuit, or thelike may be added to the pixel in FIG. 76. For example, the secondswitch 80602 may include a P-channel transistor or an n-channeltransistor, the third switch 80603 may include a transistor withpolarity opposite to that of the second switch 80602, and the secondswitch 80602 and the third switch 80603 may be controlled by the samescan line.

A structure and an operation of a pixel called a current input pixel aredescribed. A current input pixel can be applied to digital gray scaledriving and analog gray scale driving.

FIG. 77 is a diagram showing an example of a structure of a pixel calleda current input pixel.

The pixel in FIG. 77 includes a driving transistor 80700, a first switch80701, a second switch 80702, a third switch 80703, a capacitor 80704,and a light-emitting element 80730. A gate of the driving transistor80700 is connected to a signal line 80711 through the second switch80702 and the first switch 80701 in this order. Further, the gate of thedriving transistor 80700 is connected to a power supply line 80712through the capacitor 80704. A first electrode of the driving transistor80700 is connected to the power supply line 80712. A second electrode ofthe driving transistor 80700 is connected to the signal line 80711through the first switch 80701. Further, the second electrode of thedriving transistor 80700 is connected to a first electrode of thelight-emitting element 80730 through the third switch 80703. A secondelectrode of the light-emitting element 80730 corresponds to a commonelectrode 80731. Note that on/off of the first switch 80701, the secondswitch 80702, and the third switch 80703 is controlled by a signal inputto a first scan line 80713, a signal input to a second scan line 80714,and a signal input to a third scan line 80715, respectively.

A pixel structure shown in FIG. 77 is not limited to this. For example,a switch, a resistor, a capacitor, a transistor, a logic circuit, or thelike may be added to the pixel in FIG. 77. For example, the first switch80701 may include a P-channel transistor or an N-channel transistor, thesecond switch 80702 may include a transistor with the same polarity asthat of the first switch 80701, and the first switch 80701 and thesecond switch 80702 may be controlled by the same scan line. The secondswitch 80702 may be provided between the gate of the driving transistor80700 and the signal line 80711.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 16

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a display device using anorganic EL element is described.

FIG. 78A shows an example of a top plan view (a layout diagram) of apixel including two transistors. FIG. 78B shows an example of across-sectional view taken along X-X′ in FIG. 78A.

FIGS. 78A and 78B show a first transistor 60105, a first wiring 60106, asecond wiring 60107, a second transistor 60108, a third wiring 60111, acounter electrode 60112, a capacitor 60113, a pixel electrode 60115, apartition wall 60116, an organic conductive film 60117, an organic thinfilm 60118, and a substrate 60119. Note that it is preferable that thefirst transistor 60105 be used as a switching transistor, the secondtransistor 60108 as a driving transistor, the first wiring 60106 as agate signal line, the second wiring 60107 as a source signal line, andthe third wiring 60111 as a current supply line.

A gate electrode of the first transistor 60105 is electrically connectedto the first wiring 60106, one of a source electrode and a drainelectrode of the first transistor 60105 is electrically connected to thesecond wiring 60107, and the other of the source electrode or the drainelectrode of the first transistor 60105 is electrically connected to agate electrode of the second transistor 60108 and one electrode of thecapacitor 60113. Note that the gate electrode of the first transistor60105 includes a plurality of gate electrodes. Accordingly, leakagecurrent in the off state of the first transistor 60105 can be reduced.

One of a source electrode and a drain electrode of the second transistor60108 is electrically connected to the third wiring 60111, and the otherof the source electrode or the drain electrode of the second transistor60108 is electrically connected to the pixel electrode 60115.Accordingly, current flowing to the pixel electrode 60115 can becontrolled by the second transistor 60108.

The organic conductive film 60117 is provided over the pixel electrode60115, and the organic thin film 60118 (an organic compound layer) isfurther provided thereover. The counter electrode 60112 is provided overthe organic thin film 60118 (the organic compound layer). Note that thecounter electrode 60112 may be formed over a surface of all pixels to becommonly connected to all the pixels, or may be patterned using a shadowmask or the like.

Light emitted from the organic thin film 60118 (the organic compoundlayer) is transmitted through either the pixel electrode 60115 or thecounter electrode 60112.

In FIG. 78B, the case where light is emitted to the pixel electrodeside, that is, a side on which the transistor and the like are formed isreferred to as bottom emission; and the case where light is emitted tothe counter electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60115 be formed of a light-transmitting conductive film. Inthe case of top emission, it is preferable that the counter electrode60112 be formed of a light-transmitting conductive film.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surfaceuniformly and light emission of RGB can be obtained by using a colorfilter.

Note that the structures shown in FIGS. 78A and 78B are examples, andvarious structures can be employed for a pixel layout, a cross-sectionalstructure, a stacking order of electrodes of an EL element, and thelike, as well as the structures shown in FIGS. 78A and 78B. Further, asa light-emitting element, various elements such as a crystalline elementsuch as an LED, and an element formed of an inorganic thin film can beused as well as the element formed of the organic thin film shown in thedrawing.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 17

In this embodiment mode, a structure of an EL element is described. Inparticular, a structure of an organic EL element is described.

A structure of a mixed junction EL element is described. As an example,a structure is described, which includes a layer (a mixed layer) inwhich a plurality of materials among a hole injecting material, a holetransporting material, a light-emitting material, an electrontransporting material, an electron injecting material, and the like aremixed (hereinafter referred to as a mixed junction type EL element),which is different from a stacked-layer structure where a hole injectinglayer formed of a hole injecting material, a hole transporting layerformed of a hole transporting material, a light-emitting layer formed ofa light-emitting material, an electron transporting layer formed of anelectron transporting material, an electron injecting layer formed of anelectron injecting material, and the like are clearly distinguished.

FIGS. 79A to 79E are schematic views each showing a structure of a mixedjunction type EL element. Note that a layer interposed between the anode190101 and the cathode 190102 corresponds to an EL layer.

In the structure shown in FIG. 79A, the EL layer includes a holetransporting region 190103 formed of a hole transporting material and anelectron transporting region 190104 formed of an electron transportingmaterial. The hole transporting region 190103 is closer to the anodethan the electron transporting region 190104. A mixed region 190105including both the hole transporting material and the electrontransporting material is provided between the hole transporting region190103 and the electron transporting region 190104.

In the direction from the anode 190101 to the cathode 190102, aconcentration of the hole transporting material in the mixed region190105 is decreased and a concentration of the electron transportingmaterial in the mixed region 190105 is increased.

A concentration gradient can be freely set. For example, a ratio ofconcentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting layer190103 formed of only the hole transporting material. Alternatively, aratio of concentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting layer190103 formed of only the hole transporting material and the electrontransporting layer 190104 formed of only the electron transportingmaterial. A ratio of concentrations may be changed depending on adistance from the anode or the cathode. Further, the ratio ofconcentrations may be changed continuously.

A region 190106 to which a light-emitting material is added is includedin the mixed region 190105. A light emission color of the EL element canbe controlled by the light-emitting material. Further, carriers can betrapped by the tight-emitting material. As the light-emitting material,various fluorescent dyes as well as a metal complex having a quinolineskeleton, a benzooxazole skeleton, or a benzothiazole skeleton can beused. The light emission color of the EL element can be controlled byadding the light-emitting material.

As the anode 190101, an electrode material having a high work functionis preferably used in order to inject holes efficiently. For example, atransparent electrode formed of indium tin oxide (ITO), indium zincoxide (IZO), ZnO, SnO₂, In₂O₃, or the like can be used. When alight-transmitting property is not needed, the anode 190101 may beformed of an opaque metal material.

As the hole transporting material, an aromatic amine compound or thelike can be used.

As the electron transporting material, a metal complex having aquinoline derivative, 8-quinolinol, or a derivative thereof as a ligand(especially tris(8-quinolinolato)aluminum (Alq₃)), or the like can beused.

As the cathode 190102, an electrode material having a low work functionis preferably used in order to inject electrons efficiently. Forexample, a metal such as aluminum, indium, magnesium, silver, calcium,barium, or lithium can be used by itself. Alternatively, an alloy of theaforementioned metal or an alloy of the aforementioned metal and anothermetal may be used.

FIG. 79B is the schematic view of the structure of the EL element, whichis different from that of FIG. 79A. Note that portions which are thesame as those in FIG. 79A are denoted by the same reference numerals anddescription thereof is omitted.

In FIG. 79B, a region to which a light-emitting material is added is notincluded. However, when a material (an electron-transporting andlight-emitting material) having both an electron transporting propertyand a light-emitting property, for example,tris(8-quinolinolato)aluminum (Alq₃) is used as a material added to theelectron transporting region 190104, light emission can be performed.

Alternatively, as a material added to the hole transporting region190103, a material (a hole-transporting and light-emitting material)having both a hole transporting property and a light-emitting propertymay be used.

FIG. 79C is the schematic view of the structure of the EL element, whichis different from those of FIGS. 79A and 79B. Note that portions whichare the same as those in FIGS. 79A and 79B are denoted by the samereference numerals and description thereof is omitted.

In FIG. 79C, a region 190107 included in the mixed region 190105 isprovided, to which a hole blocking material having a larger energydifference between the highest occupied molecular orbital and the lowestunoccupied molecular orbital than the hole transporting material isadded. The region 190107 to which the hole blocking material is added isprovided closer to the cathode 190102 than the region 190106 to whichthe light-emitting material is added in the mixed region 190105; thus, arecombination rate of carriers and light emission efficiency can beincreased. The aforementioned structure provided with the region 190107to which the hole blocking material is added is especially effective inan EL element which utilizes light emission (phosphorescence) by atriplet exciton.

FIG. 79D is the schematic view of the structure of the EL element, whichis different from those of FIGS. 79A to 79C. Note that portions whichare the same as those in FIGS. 79A to 79C are denoted by the samereference numerals and description thereof is omitted.

In FIG. 79D, a region 190108 included in the mixed region 190105 isprovided, to which an electron blocking material having a larger energydifference between the highest occupied molecular orbital and the lowestunoccupied molecular orbital than the electron transporting material isadded. The region 190108 to which the electron blocking material isadded is provided closer to the anode 190101 than the region 190106 towhich the light-emitting material is added in the mixed region 190105;thus, a recombination rate of carriers and light emission efficiency canbe increased. The aforementioned structure provided with the region190108 to which the electron blocking material is added is especiallyeffective in an EL element which utilizes light emission(phosphorescence) by a triplet exciton.

FIG. 79E is the schematic view of the structure of the mixed junctiontype EL element, which is different from those of FIGS. 79A to 79D. FIG.79E shows an example of a structure where a region 190109 to which ametal material is added is included in part of an EL layer in contactwith an electrode of the EL element. In FIG. 79E, portions which are thesame as those in FIGS. 79A to 79D are denoted by the same referencenumerals and description thereof is omitted. In FIG. 79E, MgAg (a Mg—Agalloy) may be used as the cathode 190102, and the region 190109 to whichan Al (aluminum) alloy is added may be included in a region of theelectron transporting region 190104 to which the electron transportingmaterial is added, which is in contact with the cathode 190102, forexample. By the aforementioned structure, oxidation of the cathode canbe prevented, and electron injection efficiency from the cathode can beincreased. Therefore, the lifetime of the mixed junction type EL elementcan be extended, and a driving voltage can be lowered.

As a method of forming the aforementioned mixed junction type ELelement, a co-evaporation method or the like can be used.

In the mixed junction type EL elements as shown in FIGS. 79A to 79E, aclear interface between the layers does not exist, and chargeaccumulation can be reduced. Thus, the lifetime of the EL element can beextended, and a driving voltage can be lowered.

Note that the structures shown in FIGS. 79A to 79E can be combined witheach other.

A structure of the mixed junction type EL element is not limited tothose described above, and various structures can be freely used.

An organic material which forms an EL layer of an EL element may be alow molecular material or a high molecular material, and both of thematerials may be used. When a low molecular material is used as anorganic compound material, a film can be formed by an evaporationmethod. When a high molecular material is used as the EL layer, the highmolecular material is dissolved in a solvent and a film can be formed bya spin coating method or an ink-jet method.

The EL layer may be formed of a middle molecular material. In thisspecification, a middle molecule organic light-emitting material denotesan organic light-emitting material without a sublimation property andwith a polymerization degree of approximately 20 or less. When a middlemolecular material is used as the EL layer, a film can be formed by anink-jet method or the like.

A low molecular material, a high molecular material, and a middlemolecular material may be used in combination.

An EL element may utilize either light emission (fluorescence) by asinglet exciton or light emission (phosphorescence) by a tripletexciton.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 18

In this embodiment mode, a structure of an EL element is described. Inparticular, a structure of an inorganic EL element is described.

As a base material to be used for a light-emitting material, sulfide,oxide, or nitride can be used. As sulfide, zinc sulfide (ZnS), cadmiumsulfide (CdS), calcium sulfide (CaS), yttrium sulfide (Y₂S₃), galliumsulfide (Ga₂S₃), strontium sulfide (SrS), barium sulfide (BaS), or thelike can be used, for example. As oxide, zinc oxide (ZnO), yttrium oxide(Y₂O₃), or the like can be used, for example. As nitride, aluminumnitride (AlN), gallium nitride (GaN), indium nitride (InN), or the likecan be used, for example. Further, zinc selenide (ZnSe), zinc telluride(ZnTe), or the like; or a ternary mixed crystal such as calcium galliumsulfide (CaGa₂S₄), strontium gallium sulfide (SrGa₂S₄), or bariumgallium sulfide (BaGa₂S₄) may be used.

As a luminescence center for localized light emission, manganese (Mn),copper (Cu), samarium (Sm), terbium (Tb), erbium (Er), thulium (Tm),europium (Eu), cerium (Ce), praseodymium (Pr), or the like can be used.Further, a halogen element such as fluorine (F) or chlorine (Cl) may beadded for charge compensation.

On the other hand, as a luminescence center for donor-acceptorrecombination light emission, a light-emitting material including afirst impurity element forming a donor level and a second impurityelement forming an acceptor level can be used. As the first impurityelement, fluorine (F), chlorine (Cl), aluminum (Al), or the like can beused, for example. As the second impurity element, copper (Cu), silver(Ag), or the like can be used, for example.

FIGS. 80A to 80C each show an example of a thin-film type inorganic ELelement which can be used as a light-emitting element. In FIGS. 80A to80C, the light-emitting element includes a first electrode layer 120100,an electroluminescent layer 120102, and a second electrode layer 120103.

The light-emitting elements in FIGS. 80B and 80C each have a structurewhere an insulating film is provided between the electrode layer and theelectroluminescent layer in the light-emitting element in FIG. 80A. Thelight-emitting element in FIG. 80B includes an insulating film 120104between the first electrode layer 120100 and the electroluminescentlayer 120102. The light-emitting element in FIG. 80C includes aninsulating film 120105 between the first electrode layer 120100 and theelectroluminescent layer 120102, and an insulating film 120106 betweenthe second electrode layer 120103 and the electroluminescent layer120102. Accordingly, the insulating film may be provided between theelectroluminescent layer and one of the electrode layers interposing theelectroluminescent layer, or may be provided between theelectroluminescent layer and each of the electrode layers interposingthe electroluminescent layer. Further, the insulating film may be asingle layer or stacked layers including a plurality of layers.

FIGS. 81A to 81C each show an example of a dispersion type inorganic ELelement which can be used as a light-emitting element. A light-emittingelement in FIG. 81A has a stacked-layer structure of a first electrodelayer 120200, an electroluminescent layer 120202, and a second electrodelayer 120203. The electroluminescent layer 120202 includes alight-emitting material 120201 held by a binder.

The light-emitting elements in FIGS. 81B and 81C each have a structurewhere an insulating film is provided between the electrode layer and theelectroluminescent layer in the light-emitting element in FIG. 81A. Thelight-emitting element in FIG. 81B includes an insulating film 120204between the first electrode layer 120200 and the electroluminescentlayer 120202. The light-emitting element in FIG. 81C includes aninsulating film 120205 between the first electrode layer 120200 and theelectroluminescent layer 120202, and an insulating film 120206 betweenthe second electrode layer 120203 and the electroluminescent layer120202. Accordingly, the insulating film may be provided between theelectroluminescent layer and one of the electrode layers interposing theelectroluminescent layer, or may be provided between theelectroluminescent layer and each of the electrode layers interposingthe electroluminescent layer. Further, the insulating film may be asingle layer or stacked layers including a plurality of layers.

The insulating film 120204 is provided in contact with the firstelectrode layer 120200 in FIG. 81B; however, the insulating film 120204may be provided in contact with the second electrode layer 120203 byreversing the positions of the insulating film and theelectroluminescent layer.

It is preferable that a material which can be used for the insulatingfilms such as the insulating film 120104 in FIG. 80B and the insulatingfilm 120204 in FIG. 81B has high withstand voltage and dense filmquality. Further, the material preferably has high dielectric constant.For example, silicon oxide (SiO₂), yttrium oxide (Y₂O₃), titanium oxide(TiO₂), aluminum oxide (Al₂O₃), hafnium oxide (HfO₂), tantalum oxide(Ta₂O₅), barium titanate (BaTiO₃), strontium titanate (SrTiO₃), leadtitanate (PbTiO₃), silicon nitride (Si₃N₄), or zirconium oxide (ZrO₂);or a mixed film of those materials or a stacked-layer film including twoor more of those materials can be used. The insulating film can beformed by sputtering, evaporation, CVD, or the like. Alternatively, theinsulating film may be formed by dispersing particles of theseinsulating materials in a binder. A binder material may be formed usinga material similar to that of a binder contained in theelectroluminescent layer, by using a method similar thereto. Thethickness of the insulating film is not particularly limited, butpreferably in the range of 10 to 1000 nm.

The light-emitting element can emit light when a voltage is appliedbetween the pair of electrode layers interposing the electroluminescentlayer. The light-emitting element can operate with DC drive or AC drive.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 19

In this embodiment mode, an example of a display device is described. Inparticular, the case where a display device is optically treated isdescribed.

A rear projection display device 130100 in FIGS. 82A and 82B is providedwith a projector unit 130111, a mirror 130112, and a screen panel130101. The rear projection display device 130100 may also be providedwith a speaker 130102 and operation switches 130104. The projector unit130111 is provided at a lower portion of a housing 130110 of the rearprojection display device 130100, and projects incident light forprojecting an image based on an image signal to the mirror 130112. Therear projection display device 130100 displays an image projected from arear surface of the screen panel 130101.

FIG. 83 shows a front projection display device 130200. The frontprojection display device 130200 is provided with the projector unit130111 and a projection optical system 130201. The projection opticalsystem 130201 projects an image to a screen or the like provided at thefront.

Hereinafter, a structure of the projector unit 130111 which is appliedto the rear projection display device 130100 in FIGS. 82A and 82B andthe front projection display device 130200 in FIG. 83 is described.

FIG. 84 shows a structure example of the projector unit 130111. Theprojector unit 130111 is provided with a light source unit 130301 and amodulation unit 130304. The light source unit 130301 is provided with alight source optical system 130303 including lenses and a light sourcelamp 130302. The light source lamp 130302 is stored in a housing so thatstray light is not scattered. As the light source lamp 130302, ahigh-pressure mercury lamp or a xenon lamp, for example, which can emita large amount of light is used. The light source optical system 130303is provided with an optical lens, a film having a function to polarizelight, a film for adjusting phase difference, an IR film, or the like asappropriate. The light source unit 130301 is provided so that incidentlight is incident on the modulation unit 130304. The modulation unit130304 is provided with a plurality of display panels 130308, a colorfilter, a dichroic mirror 130305, a total reflection mirror 130306, aretardation plate 130307, a prism 130309, and a projection opticalsystem 130310. Light emitted from the light source unit 130301 is splitinto a plurality of optical paths by the dichroic mirror 130305.

Each optical path is provided with a color filter which transmits lightwith a predetermined wavelength or wavelength range and the displaypanel 130308. The transmissive display panel 130308 modulatestransmitted light based on an image signal. Light of each colortransmitted through the display panel 130308 is incident on the prism130309, and an image is displayed on the screen through the projectionoptical system 130310. Note that a Fresnel lens may be provided betweenthe mirror and the screen. Projected light which is projected by theprojector unit 130111 and reflected by the mirror is converted intogenerally parallel light by the Fresnel lens to be projected on thescreen. Displacement between a chief ray and an optical axis ispreferably ±10° or less, and more preferably, ±5° or less.

The projector unit 130111 shown in FIG. 85 includes reflective displaypanels 130407, 130408, and 130409.

The projector unit 130111 in FIG. 85 includes the light source unit130301 and a modulation unit 130400. The light source unit 130301 mayhave a structure similar to that in FIG. 84. Light from the light sourceunit 130301 is split into a plurality of optical paths by dichroicmirrors 130401 and 130402 and a total reflection mirror 130403 to beincident on polarization beam splitters 130404, 130405, and 130406. Thepolarization beam splitters 130404, 130405, and 130406 are providedcorresponding to the reflective display panels 130407, 130408, and130409 which correspond to respective colors. The reflective displaypanels 130407, 130408, and 130409 modulate reflected light based on animage signal. Light of each color, which are reflected by the reflectivedisplay panels 130407, 130408, and 130409, is incident on a prism 130410to be composed, and projected through a projection optical system130411.

Among light emitted from the light source unit 130301, only light in awavelength region of red is transmitted through the dichroic mirror130401 and light in wavelength regions of green and blue is reflected bythe dichroic mirror 130401. Further, only the light in the wavelengthregion of green is reflected by the dichroic mirror 130402. The light inthe wavelength region of red, which is transmitted through the dichroicmirror 130401, is reflected by the total reflection mirror 130403 andincident on the polarization beam splitter 130404. The light in thewavelength region of blue is incident on the polarization beam splitter130405. The light in the wavelength region of green is incident on thepolarization beam splitter 130406. The polarization beam splitters130404, 130405, and 130406 have a function to split incident light intoP-polarized light and S-polarized light and a function to transmit onlyP-polarized light. The reflective display panels 130407, 130408, and130409 polarize incident light based on an image signal.

Only the S-polarized light corresponding to each color is incident onthe reflective display panels 130407, 130408, and 130409 correspondingto each color. Note that the reflective display panels 130407, 130408,and 130409 may be liquid crystal panels. In this case, the liquidcrystal panel operates in an electrically controlled birefringence (ECB)mode. Liquid crystal molecules are vertically aligned at an angle to asubstrate. Accordingly, in the reflective display panels 130407, 130408,and 130409, when a pixel is turned off, display molecules are alignednot to change a polarization state of incident light so as to reflectthe incident light. When the pixel is turned on, alignment of thedisplay molecules is changed, and the polarization state of the incidentlight is changed.

The projector unit 130111 shown in FIG. 85 can be applied to the rearprojection display device 130100 in FIGS. 82A and 82B and the frontprojection display device 130200 in FIG. 83.

FIGS. 86A to 86C each show a single-panel type projector unit. Theprojector unit 130111 shown in FIG. 86A is provided with the lightsource unit 130301, a display panel 130507, a projection optical system130511, and a retardation plate 130504. The projection optical system130511 includes one or a plurality of lenses. The display panel 130507may be provided with a color filter.

FIG. 86B shows a structure of the projector unit 130111 operating in afield sequential mode. The field sequential mode corresponds to a modein which color display is performed by light of respective colors suchas red, green, and blue sequentially incident on a display panel with atime lag, without a color filter. A higher-definition image can bedisplayed particularly by combination with a display panel withhigh-speed response to change in input signal. The projector unit 130111in FIG. 86B is provided with a rotating color filter plate 130505including a plurality of color filters with red, green, blue, or thelike between the light source unit 130301 and a display panel 130508.

FIG. 86C shows a structure of the projector unit 130111 with a colorseparation system using a micro lens, as a color display method. Thecolor separation system corresponds to a system in which color displayis realized by providing a micro lens array 130506 on the side of adisplay panel 130509, on which light is incident, and light of eachcolor is emitted from each direction. The projector unit 130111employing this system has little loss of light due to a color filter, sothat light from the light source unit 130301 can be efficientlyutilized. The projector unit 130111 in FIG. 86C is provided withdichroic mirrors 130501, 130502, and 130503 so that light of each coloris emitted to the display panel 130509 from each direction.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 20

In this embodiment mode, examples of electronic devices are described.

FIG. 87 shows a display panel module combining a display panel 900101and a circuit board 900111. The display panel 900101 includes a pixelportion 900102, a scan line driver circuit 900103, and a signal linedriver circuit 900104. The circuit board 900111 is provided with acontrol circuit 900112, a signal dividing circuit 900113, and the like,for example. The display panel 900101 and the circuit board 900111 areconnected to each other by a connection wiring 900114. An FPC or thelike can be used as the connection wiring.

FIG. 92 is a block diagram showing a main structure of a televisionreceiver. A tuner 900201 receives an image signal and an audio signal.The image signals are processed by an image signal amplifier circuit900202; an image signal processing circuit 900203 which converts asignal output from the image signal amplifier circuit 900202 into acolor signal corresponding to each color of red, green and blue; and acontrol circuit 900212 which converts the image signal into the inputspecification of a driver circuit. The control circuit 900212 outputs asignal to each of a scan line driver circuit 900214 and a signal linedriver circuit 900204. The scan line driver circuit 900214 and thesignal line driver circuit 900204 drive a display panel 900211. Whenperforming digital drive, a structure may be employed in which a signaldividing circuit 900213 is provided on the signal line side so that aninput digital signal is divided into m signals (m corresponds to apositive integer) to be supplied.

Among the signals received by the tuner 900201, an audio signal istransmitted to an audio signal amplifier circuit 900205, and an outputthereof is supplied to a speaker 900207 through an audio signalprocessing circuit 900206. A control circuit 900208 receives controlinformation on receiving station (receiving frequency) and volume froman input portion 900209 and transmits signals to the tuner 900201 or theaudio signal processing circuit 900206.

FIG. 93A shows a television receiver incorporated with a display panelmodule, which is different from FIG. 92. In FIG. 93A, a display screen900302 incorporated in a housing 900301 is formed using the displaypanel module. Note that speakers 900303, input means (an operation key900304, a connection terminal 900305, a sensor 900306 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 900307), and the like may be provided as appropriate.

FIG. 93B shows a television receiver in which only a display can becarried wirelessly. The television receiver is provided with a displayportion 900313, a speaker portion 900317, input means (an operation key900316, a connection terminal 900318, a sensor 900319 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 900320), and the like as appropriate. A battery and a signalreceiver are incorporated in a housing 900312. The battery drives thedisplay portion 900313, the speaker portion 900317, the sensor 900319,and the microphone 900320. The battery can be repeatedly charged by acharger 900310. The charger 900310 can transmit and receive an imagesignal and transmit the image signal to the signal receiver of thedisplay. The device in FIG. 93B is controlled by the operation key900316. Alternatively, the device in FIG. 93B can transmit a signal tothe charger 900310 by operating the operation key 900316. That is, thedevice may be an image and audio interactive communication device.Further alternatively, by operating the operation key 900316, the devicein FIG. 93B may transmit a signal to the charger 900310 and anotherelectronic device is made to receive a signal which can be transmittedfrom the charger 900310; thus, the device in FIG. 93B can controlcommunication of another electronic device. That is, the device may be ageneral-purpose remote control device. Note that the contents (or partthereof) described in each drawing of this embodiment mode can beapplied to the display portion 900313.

Next, a structure example of a mobile phone is described with referenceto FIG. 94.

A display panel 900501 is detachably incorporated in a housing 900530.The shape and size of the housing 900530 can be changed as appropriatein accordance with the size of the display panel 900501. The housing900530 which fixes the display panel 900501 is fitted in a printedwiring board 900531 to be assembled as a module.

The display panel 900501 is connected to the printed wiring board 900531through an FPC 900513. The printed wiring board 900531 is provided witha speaker 900532, a microphone 900533, a transmitting/receiving circuit900534, a signal processing circuit 900535 including a CPU, acontroller, and the like, and a sensor 900541 (having a function tomeasure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray). Such a module,an operation key 900536, a battery 900537, and an antenna 900540 arecombined and stored in a housing 900539. A pixel portion of the displaypanel 900501 is provided to be seen from an opening window formed in thehousing 900539.

In the display panel 900501, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed over the same substrate byusing transistors, and another part of the peripheral driver circuits (adriver circuit having high operation frequency among the plurality ofdriver circuits) may be formed over an IC chip. Then, the IC chip may bemounted on the display panel 900501 by COG (Chip On Glass).Alternatively, the IC chip may be connected to a glass substrate byusing TAB (Tape Automated Bonding) or a printed wiring board. With sucha structure, power consumption of a display device can be reduced andoperation time of the mobile phone per charge can be extended. Further,reduction in cost of the mobile phone can be realized.

The mobile phone in FIG. 94 has various functions such as, but notlimited to, a function to display various kinds of information (e.g., astill image, a moving image, and a text image); a function to display acalendar, a date, the time, and the like on a display portion; afunction to operate or edit the information displaying on the displayportion; a function to control processing by various kinds of software(programs); a function of wireless communication; a function tocommunicate with another mobile phone, a fixed phone, or an audiocommunication device by using the wireless communication function; afunction to connect with various computer networks by using the wirelesscommunication function; a function to transmit or receive various kindsof data by using the wireless communication function; a function tooperate a vibrator in accordance with incoming call, reception of data,or an alarm; and a function to generate a sound in accordance withincoming call, reception of data, or an alarm.

FIG. 95A shows a display, which includes a housing 900711, a supportbase 900712, a display portion 900713, a speaker 900717, an LED lamp900719, input means (a connection terminal 900714, a sensor 900715(having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), a microphone 900716, and an operation key 900718), andthe like. The display shown in FIG. 95A can have various functions suchas, but not limited to, a function to display various kinds ofinformation (e.g., a still image, a moving image, and a text image) onthe display portion.

FIG. 95B shows a camera, which includes a main body 900731, a displayportion 900732, a shutter button 900736, a speaker 900740, an LED lamp900741, input means (an image receiving portion 900733, operation keys900734, an external connection port 900735, a connection terminal900737, a sensor 900738 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 900739), and thelike. The camera shown in FIG. 95B can have various functions such as,but not limited to, a function to photograph a still image or a movingimage; a function to automatically adjust the photographed image (stillimage or moving image); a function to store the photographed image in arecording medium (provided externally or incorporated in the camera);and a function to display the photographed image on the display portion.

FIG. 95C shows a computer, which includes a main body 900751, a housing900752, a display portion 900753, a speaker 900760, an LED lamp 900761,a reader/writer 900762, input means (a keyboard 900754, an externalconnection port 900755, a pointing device 900756, a connection terminal900757, a sensor 900758 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 900759), and thelike. The computer shown in FIG. 95C can have various functions such as,but not limited to, a function to display various kinds of information(e.g., a still image, a moving image, and a text image) on the displayportion; a function to control processing by various kinds of software(programs); a communication function such as wireless communication orwire communication; a function to connect with various computer networksby using the communication function; and a function to transmit orreceive various kinds of data by using the communication function.

FIG. 102A shows a mobile computer, which includes a main body 901411, adisplay portion 901412, a switch 901413, a speaker 901419, an LED lamp901420, input means (operation keys 901414, an infrared port 901415, aconnection terminal 901416, a sensor 901417 (having a function tomeasure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901418), and the like. The mobile computer shown in FIG. 102Acan have various functions such as, but not limited to, a function todisplay various kinds of information (e.g., a still image, a movingimage, and a text image) on a display portion; a touch panel functionprovided on the display portion; a function to display a calendar, adate, the time, and the like on the display portion; a function tocontrol processing by various kinds of software (programs); a functionof wireless communication; a function to connect with various computernetworks by using the wireless communication function; and a function totransmit or receive various kinds of data by using the wirelesscommunication function.

FIG. 102B shows a portable image reproducing device having a recordingmedium (e.g., a DVD player), which includes a main body 901431, ahousing 901432, a display portion A 901433, a display portion B 901434,a speaker portion 901437, an LED lamp 901441, input means (a recordingmedium (e.g., a DVD) reading portion 901435, operation keys 901436, aconnection terminal 901438, a sensor 901439 (having a function tomeasure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901440), and the like. The display portion A 901433 mainlydisplays image information and the display portion B 901434 mainlydisplays text information.

FIG. 102C shows a goggle-type display, which includes a main body901451, a display portion 901452, an earphone 901453, a support portion901454, an LED lamp 901459, a speaker 901458, input means (a connectionterminal 901455, a sensor 901456 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 901457), and thelike. The goggle-type display shown in FIG. 102C can have variousfunctions such as, but not limited to, a function to display anexternally obtained image (e.g., a still image, a moving image, and atext image) on the display portion.

FIG. 103A shows a portable game machine, which includes a housing901511, a display portion 901512, a speaker portion 901513, a recordingmedium insert portion 901515, an LED lamp 901519, input means (anoperation key 901514, a connection terminal 901516, a sensor 901517(having a function to measure power, displacement, position, speed,acceleration, angular velocity, the number of rotations, distance,light, liquid, magnetism, temperature, a chemical substance, sound,time, hardness, an electric field, current, voltage, electric power,radiation, a flow rate, humidity, gradient, oscillation, smell, orinfrared ray), and a microphone 901518), and the like. The portable gamemachine shown in FIG. 103A can have various functions such as, but notlimited to, a function to read a program or data stored in the recordingmedium to display on the display portion; and a function to shareinformation by wireless communication with another portable gamemachine.

FIG. 103B shows a digital camera having a television reception function,which includes a housing 901531, a display portion 901532, a speaker901534, a shutter button 901535, an LED lamp 901541, input means (anoperation key 901533, an image receiving portion 901536, an antenna901537, a connection terminal 901538, a sensor 901539 (having a functionto measure power, displacement, position, speed, acceleration, angularvelocity, the number of rotations, distance, light, liquid, magnetism,temperature, a chemical substance, sound, time, hardness, an electricfield, current, voltage, electric power, radiation, a flow rate,humidity, gradient, oscillation, smell, or infrared ray), and amicrophone 901540), and the like. The digital camera having a televisionreception function shown in FIG. 103B can have various functions suchas, but not limited to, a function to photograph a still image or amoving image; a function to automatically adjust the photographed image;a function to obtain various kinds of information from the antenna; afunction to store the photographed image or the information obtainedfrom the antenna; and a function to display the photographed image orthe information obtained from the antenna on the display portion.

FIG. 104 shows a portable game machine, which includes a housing 901611,a first display portion 901612, a second display portion 901613, aspeaker portion 901614, a recording medium insert portion 901616, an LEDlamp 901620, input means (an operation key 901615, a connection terminal901617, a sensor 901418 (having a function to measure power,displacement, position, speed, acceleration, angular velocity, thenumber of rotations, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, an electric field, current,voltage, electric power, radiation, a flow rate, humidity, gradient,oscillation, smell, or infrared ray), and a microphone 901619), and thelike. The portable game machine shown in FIG. 104 can have variousfunctions such as, but not limited to, a function to read a program ordata stored in the recording medium to display on the display portion;and a function to share information by wireless communication withanother portable game machine.

As shown in FIGS. 95A to 95C, 102A to 102C, 103A to 103C, and 104, theelectronic device includes a display portion for displaying some kind ofinformation.

Next, application examples of a semiconductor device are described.

FIG. 96 shows an example where a semiconductor device is incorporated ina constructed object. FIG. 96 shows a housing 900810, a display portion900811, a remote control device 900812 which is an operation portion, aspeaker portion 900813, and the like. The semiconductor device isincorporated in the constructed object as a wall-hanging type and can beprovided without requiring a large space.

FIG. 97 shows another example where a semiconductor device isincorporated in a constructed object. A display panel 900901 isincorporated with a prefabricated bath 900902, and a person who takes abath can view the display panel 900901. The display panel 900901 has afunction to display information by an operation by a person who takes abath; and a function to be used as an advertisement or an entertainmentmeans.

The semiconductor device can be provided not only to a side wall of theprefabricated bath 900902 as shown in FIG. 97, but also to variousplaces. For example, the semiconductor device can be incorporated withpart of a mirror, a bathtub itself, or the like. At this time, a shapeof the display panel 900901 may be changed in accordance with a shape ofthe mirror or the bathtub.

FIG. 98 shows another example where a semiconductor device isincorporated in a constructed object. A display panel 901002 is bent andattached to a curved surface of a column-shaped object 901001. Here, autility pole is described as the column-shaped object 901001.

The display panel 901002 shown in FIG. 98 is provided at a positionhigher than a human viewpoint. When the same images are displayed on thedisplay panels 901002 provided in constructed objects which standtogether in large numbers outdoors, such as utility poles, advertisementcan be performed to unspecified number of viewers. Since it is easy forthe display panel 901002 to display the same images and instantly switchimages by external control, highly effective information display andadvertisement effect can be expected. When provided with self-luminousdisplay elements, the display panel 901002 can be effectively used as ahighly visible display medium even at night. When the display panel901002 is provided in the utility pole, a power supply means for thedisplay panel 901002 can be easily obtained. In an emergency such asdisaster, the display panel 901002 can also be used as a means torapidly transmit correct information to victims.

As the display panel 901002, a display panel in which a switchingelement such as an organic transistor is provided over a film-shapedsubstrate, and a display element is driven, so that an image can bedisplayed can be used, for example.

In this embodiment mode, a wall, a column-shaped object, and aprefabricated bath are shown as examples of a constructed object;however, this embodiment mode is not limited thereto, and variousconstructed objects can be provided with a semiconductor device.

Next, examples where a semiconductor device is incorporated with amoving object are described.

FIG. 99 shows an example where a semiconductor device is incorporatedwith a car. A display panel 901102 is incorporated with a car body901101, and can display an operation of the car body or informationinput from inside or outside the car body on demand. Note that anavigation function may be provided.

The semiconductor device can be provided not only to the car body 901101as shown in FIG. 99, but also to various places. For example, thesemiconductor device can be incorporated with a glass window, a door, asteering wheel, a gear shift, a seat, a rear-view mirror, and the like.At this time, a shape of the display panel 901102 may be changed inaccordance with a shape of an object provided with the semiconductordevice.

FIGS. 100A and 100B show examples where a semiconductor device isincorporated with a train car are described.

FIG. 100A shows an example where a display panel 901202 is provided inglass of a door 901201 in a train car, which has an advantage comparedwith a conventional advertisement using paper in that labor cost forchanging an advertisement is not necessary. Since the display panel901202 can instantly switch images displaying on a display portion by anexternal signal, images on the display panel can be switched in everytime period when types of passengers on the train are changed, forexample; thus, more effective advertisement effect can be expected.

FIG. 100B shows an example where the display panels 901202 are providedto a glass window 901203 and a ceiling 901204 as well as the glass ofthe door 901201 in the train car. In this manner, the semiconductordevice can be easily provided to a place where the semiconductor devicehas been difficult to be provided conventionally; thus, effectiveadvertisement effect can be obtained. Further, the semiconductor devicecan instantly switch images displayed on a display portion by anexternal signal; thus, cost and time for changing an advertisement canbe reduced, and more flexible advertisement management and informationtransmission can be realized.

The semiconductor device can be provided not only to the door 901201,the glass window 901203, and the ceiling 901204 as shown in FIG. 100,but also to various places. For example, the semiconductor device can beincorporated with a strap, a seat, a handrail, a floor, and the like. Atthis time, a shape of the display panel 901202 may be changed inaccordance with a shape of an object provided with the semiconductordevice.

FIGS. 101A and 101B show an example where a semiconductor device isincorporated with a passenger airplane.

FIG. 101A shows a shape of a display panel 901302 attached to a ceiling901301 above a seat of the passenger airplane when the display panel901302 is used. The display panel 901302 is incorporated with theceiling 901301 using a hinge portion 901303, and the passenger can viewthe display panel 901302 by stretching of the hinge portion 901303. Thedisplay panel 901302 has a function to display information by anoperation by the passenger and a function to be used as an advertisementor an entertainment means. In addition, when the hinge portion is bentand put in the ceiling 901301 of the airplane as shown in FIG. 101B,safety in taking-off and landing can be assured. Note that when adisplay element in the display panel is lit in an emergency, the displaypanel can also be used as an information transmission means and anevacuation light.

The semiconductor device can be provided not only to the ceiling 901301as shown in FIGS. 101A and 101B, but also to various places. Forexample, the semiconductor device can be incorporated with a seat, atable attached to a seat, an armrest, a window, and the like. A largedisplay panel which a large number of people can view may be provided ata wall of an airframe. At this time, a shape of the display panel 901302may be changed in accordance with a shape of an object provided with thesemiconductor device.

Note that in this embodiment mode, bodies of a train car, a car, and anairplane are shown as a moving object; however, the invention is notlimited thereto, and a semiconductor device can be provided to variousobjects such as a motorcycle, an four-wheel drive car (including a car,a bus, and the like), a train (including a monorail, a railroad car, andthe like), and a vessel. Since a semiconductor device can instantlyswitch images displayed on a display panel in a moving object by anexternal signal, a moving object is provided with the semiconductordevice, so that the moving object can be used as an advertisementdisplay board for an unspecified number of customers, an informationdisplay board in disaster, and the like.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

Similarly, the contents (or may be part of the contents) described ineach drawing of this embodiment mode can be freely applied to, combinedwith, or replaced with the contents (or may be part of the contents)described in a drawing in another embodiment mode. Further, even moredrawings can be formed by combining each part with part of anotherembodiment mode in the drawings of this embodiment mode.

Note that this embodiment mode shows an example of an embodied case ofthe contents (or may be part of the contents) described in otherembodiment modes, an example of slight transformation thereof, anexample of partial modification thereof, an example of improvementthereof, an example of detailed description thereof, an applicationexample thereof, an example of related part thereof, or the like.Therefore, the contents described in other embodiment modes can befreely applied to, combined with, or replaced with this embodiment mode.

Embodiment Mode 21

As described above, the following inventions are at least included inthis specification.

A liquid crystal display device includes a pixel having a liquid crystalelement, and a driver circuit. The driver circuit includes a firsttransistor, a second transistor, a third transistor, a fourthtransistor, a fifth transistor, a sixth transistor, a seventhtransistor, and an eighth transistor. A first electrode of the firsttransistor is electrically connected to a fourth wiring and a secondelectrode of the first transistor is electrically connected to a thirdwiring. A first electrode of the second transistor is electricallyconnected to a seventh wiring; a second electrode of the secondtransistor is electrically connected to the third wiring; and a gateelectrode of the second transistor is electrically connected to a fifthwiring. A first electrode of the third transistor is electricallyconnected to a sixth wiring; a second electrode of the third transistoris electrically connected to a gate electrode of the sixth transistor;and a gate electrode of the third transistor is electrically connectedto the fourth wiring. A first electrode of the fourth transistor iselectrically connected to the seventh wiring; a second electrode of thefourth transistor is electrically connected to the gate electrode of thesixth transistor; and a gate electrode of the fourth transistor iselectrically connected to the fifth wiring. A first electrode of thefifth transistor is electrically connected to the sixth wiring; a secondelectrode of the fifth transistor is electrically connected to a gateelectrode of the first transistor; and a gate electrode of the fifthtransistor is electrically connected to a first wiring. A firstelectrode of the sixth transistor is electrically connected to theseventh wiring and a second electrode of the sixth transistor iselectrically connected to the gate electrode of the first transistor. Afirst electrode of the seventh transistor is electrically connected tothe seventh wiring; a second electrode of the seventh transistor iselectrically connected to the gate electrode of the first transistor;and a gate electrode of the seventh transistor is electrically connectedto a second wiring. A first electrode of the eighth transistor iselectrically connected to the seventh wiring; a second electrode of theeighth transistor is electrically connected to the gate electrode of thesixth transistor; and a gate electrode of the eighth transistor iselectrically connected to the gate electrode of the first transistor.

In the above-described structure, the first transistor can be formed soas to have the largest value of W/L (a ratio of a channel width W to achannel length L) among the first to eighth transistors. In addition,the value of W/L of the first transistor may be twice to five times avalue of W/L of the fifth transistor. Further, channel length L of thethird transistor may be longer than channel length L of the eighthtransistor. Furthermore, a capacitor may be provided between the secondelectrode and the gate electrode of the first transistor. Moreover, thefirst to eighth transistors may be N-channel transistors. The first toeighth transistors may be formed by using amorphous silicon.

A liquid crystal display device includes a pixel having a liquid crystalelement, a first driver circuit, and a second driver circuit. The firstdriver circuit includes a first transistor, a second transistor, a thirdtransistor, a fourth transistor, a fifth transistor, a sixth transistor,a seventh transistor, and an eighth transistor. A first electrode of thefirst transistor is electrically connected to a fourth wiring and asecond electrode of the first transistor is electrically connected to athird wiring. A first electrode of the second transistor is electricallyconnected to a seventh wiring; a second electrode of the secondtransistor is electrically connected to the third wiring; and a gateelectrode of the second transistor is electrically connected to a fifthwiring. A first electrode of the third transistor is electricallyconnected to a sixth wiring; a second electrode of the third transistoris electrically connected to a gate electrode of the sixth transistor;and a gate electrode of the third transistor is electrically connectedto the fourth wiring. A first electrode of the fourth transistor iselectrically connected to the seventh wiring; a second electrode of thefourth transistor is electrically connected to the gate electrode of thesixth transistor; and a gate electrode of the fourth transistor iselectrically connected to the fifth wiring. A first electrode of thefifth transistor is electrically connected to the sixth wiring; a secondelectrode of the fifth transistor is electrically connected to a gateelectrode of the first transistor; and a gate electrode of the fifthtransistor is electrically connected to a first wiring. A firstelectrode of the sixth transistor is electrically connected to theseventh wiring and a second electrode of the sixth transistor iselectrically connected to the gate electrode of the first transistor. Afirst electrode of the seventh transistor is electrically connected tothe seventh wiring; a second electrode of the seventh transistor iselectrically connected to the gate electrode of the first transistor;and a gate electrode of the seventh transistor is electrically connectedto a second wiring. A first electrode of the eighth transistor iselectrically connected to the seventh wiring; a second electrode of theeighth transistor is electrically connected to the gate electrode of thesixth transistor; and a gate electrode of the eighth transistor iselectrically connected to the gate electrode of the first transistor.The second driver circuit includes a ninth transistor, a tenthtransistor, an eleventh transistor, a twelfth transistor, a thirteenthtransistor, a fourteenth transistor, a fifteenth transistor, and asixteenth transistor. A first electrode of the ninth transistor iselectrically connected to an eleventh wiring and a second electrode ofthe ninth transistor is electrically connected to a tenth wiring. Afirst electrode of the tenth transistor is electrically connected to afourteenth wiring; a second electrode of the tenth transistor iselectrically connected to the tenth wiring; and a gate electrode of thetenth transistor is electrically connected to a twelfth wiring. A firstelectrode of the eleventh transistor is electrically connected to athirteenth wiring; a second electrode of the eleventh transistor iselectrically connected to a gate electrode of the fourteenth transistor;and a gate electrode of the eleventh transistor is electricallyconnected to the eleventh wiring. A first electrode of the twelfthtransistor is electrically connected to the fourteenth wiring; a secondelectrode of the twelfth transistor is electrically connected to thegate electrode of the fourteenth transistor; and a gate electrode of thetwelfth transistor is electrically connected to the twelfth wiring. Afirst electrode of the thirteenth transistor is electrically connectedto the thirteenth wiring; a second electrode of the thirteenthtransistor is electrically connected to a gate electrode of the ninthtransistor; and a gate electrode of the thirteenth transistor iselectrically connected to an eighth wiring. A first electrode of thefourteenth transistor is electrically connected to the fourteenth wiringand a second electrode of the fourteenth transistor is electricallyconnected to the gate electrode of the ninth transistor. A firstelectrode of the fifteenth transistor is electrically connected to thefourteenth wiring; a second electrode of the fifteenth transistor iselectrically connected to the gate electrode of the ninth transistor;and a gate electrode of the fifteenth transistor is electricallyconnected to a ninth wiring. A first electrode of the sixteenthtransistor is electrically connected to the fourteenth wiring; a secondelectrode of the sixteenth transistor is electrically connected to thegate electrode of the fourteenth transistor; and a gate electrode of thesixteenth transistor is electrically connected to the gate electrode ofthe ninth transistor.

The fourth wiring and the eleventh wiring may be electrically connected;the fifth wiring and the twelfth wiring may be electrically connected;the sixth wiring and the thirteenth wiring may be electricallyconnected; and the seventh wiring and the fourteenth wiring may beelectrically connected. The fourth wiring and the eleventh wiring may bethe same wiring; the fifth wiring and the twelfth wiring may be the samewiring; the sixth wiring and the thirteenth wiring may be the samewiring; and the seventh wiring and the fourteenth wiring may be the samewiring. The third wiring and the tenth wiring may be electricallyconnected. The third wiring and the tenth wiring may be the same wiring.In addition, The first transistor may be formed so as to have thelargest value of W/L (a ratio of a channel width W to a channel lengthL) among the first to eighth transistors, and the ninth transistor maybe formed so as to have the largest value of W/L (a ratio of a channelwidth W to a channel length L) among the ninth to sixteenth transistors.Further, the value of W/L of the first transistor may be twice to fivetimes a value of W/L of the fifth transistor, and the value of W/L ofthe ninth transistor may be twice to five times a value of W/L of thetwelfth transistor. Furthermore, channel length L of the thirdtransistor may be longer than channel length L of the eighth transistor,and channel length L of the eleventh transistor may be longer thanchannel length L of the sixteenth transistor. Moreover, a capacitor maybe provided between the second electrode and the gate electrode of thefirst transistor, and a capacitor may be provided between the secondelectrode and the gate electrode of the ninth transistor. The first tosixteenth transistors may be N-channel transistors. The first tosixteenth transistors may use amorphous silicon as semiconductor layers.

Each of the liquid crystal display device shown in this embodiment modecorresponds to the liquid crystal display device described in thisspecification. Therefore, operation effects which are similar to thoseof other embodiment modes is obtained.

This application is based on Japanese Patent Application serial No.2006-270016 filed in Japan Patent Office on Sep. 29, 2006, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a pixel portion; anda driver circuit electrically connected to the pixel portion, the drivercircuit comprising: a first transistor; a second transistor; a thirdtransistor; a fourth transistor; a fifth transistor; a sixth transistor;a seventh transistor; and an eighth transistor, wherein one of a sourceand a drain of the first transistor is electrically connected to awiring, wherein one of a source and a drain of the second transistor iselectrically connected to the wiring, wherein one of a source and adrain of the third transistor is electrically connected to a gate of thesecond transistor, wherein one of a source and a drain of the fourthtransistor is electrically connected to the gate of the secondtransistor, wherein one of a source and a drain of the fifth transistoris electrically connected to a gate of the first transistor, wherein oneof a source and a drain of the sixth transistor is electricallyconnected to the gate of the first transistor, wherein a gate of thesixth transistor is electrically connected to the gate of the secondtransistor, wherein one of a source and a drain of the seventhtransistor is electrically connected to the gate of the firsttransistor, wherein one of a source and a drain of the eighth transistoris electrically connected to the gate of the second transistor, whereina gate of the eighth transistor is electrically connected to the gate ofthe first transistor, wherein a first signal is input to the other ofthe source and the drain of the first transistor, wherein a secondsignal is input to a gate of the fourth transistor, wherein a potentialis supplied to the other of the source and the drain of the secondtransistor, wherein the potential is supplied to the other of the sourceand the drain of the fourth transistor, wherein the potential issupplied to the other of the source and the drain of the sixthtransistor, wherein the potential is supplied to the other of the sourceand the drain of the eighth transistor, and wherein an output signal isoutput from the wiring.
 2. The display device according to claim 1,wherein the one of the source and the drain of the fifth transistor isdirectly connected to the gate of the first transistor.
 3. The displaydevice according to claim 1, wherein the one of the source and the drainof the sixth transistor is directly connected to the gate of the firsttransistor.
 4. The display device according to claim 1, wherein the oneof the source and the drain of the seventh transistor is directlyconnected to the gate of the first transistor.
 5. The display deviceaccording to claim 1, wherein the gate of the eighth transistor isdirectly connected to the gate of the first transistor.
 6. A displaydevice comprising: a pixel portion; and a driver circuit electricallyconnected to the pixel portion, the driver circuit comprising: a firsttransistor; a second transistor; a third transistor; a fourthtransistor; a fifth transistor; a sixth transistor; a seventhtransistor; and an eighth transistor, wherein one of a source and adrain of the first transistor is electrically connected to a wiring,wherein one of a source and a drain of the second transistor iselectrically connected to the wiring, wherein one of a source and adrain of the third transistor is electrically connected to a gate of thesecond transistor, wherein one of a source and a drain of the fourthtransistor is electrically connected to the gate of the secondtransistor, wherein one of a source and a drain of the fifth transistoris electrically connected to a gate of the first transistor, wherein oneof a source and a drain of the sixth transistor is electricallyconnected to the gate of the first transistor, wherein a gate of thesixth transistor is electrically connected to the gate of the secondtransistor, wherein one of a source and a drain of the seventhtransistor is electrically connected to the gate of the firsttransistor, wherein one of a source and a drain of the eighth transistoris electrically connected to the gate of the second transistor, whereina gate of the eighth transistor is electrically connected to the gate ofthe first transistor, wherein a first signal is input to the other ofthe source and the drain of the first transistor, wherein a secondsignal is input to a gate of the fourth transistor, wherein a thirdsignal is input to a gate of the third transistor, wherein a potentialis supplied to the other of the source and the drain of the secondtransistor, wherein the potential is supplied to the other of the sourceand the drain of the fourth transistor, wherein the potential issupplied to the other of the source and the drain of the sixthtransistor, wherein the potential is supplied to the other of the sourceand the drain of the eighth transistor, and wherein an output signal isoutput from the wiring.
 7. The display device according to claim 6,wherein the one of the source and the drain of the fifth transistor isdirectly connected to the gate of the first transistor.
 8. The displaydevice according to claim 6, wherein the one of the source and the drainof the sixth transistor is directly connected to the gate of the firsttransistor.
 9. The display device according to claim 6, wherein the oneof the source and the drain of the seventh transistor is directlyconnected to the gate of the first transistor.
 10. The display deviceaccording to claim 6, wherein the gate of the eighth transistor isdirectly connected to the gate of the first transistor.
 11. A displaydevice comprising: a pixel portion; and a driver circuit electricallyconnected to the pixel portion, the driver circuit comprising: a firsttransistor; a second transistor; a third transistor; a fourthtransistor; a fifth transistor; a sixth transistor; a seventhtransistor; an eighth transistor; and a ninth transistor, wherein one ofa source and a drain of the first transistor is electrically connectedto a wiring, wherein one of a source and a drain of the secondtransistor is electrically connected to the wiring, wherein one of asource and a drain of the third transistor is electrically connected toa gate of the second transistor, wherein one of a source and a drain ofthe fourth transistor is electrically connected to the gate of thesecond transistor, wherein one of a source and a drain of the fifthtransistor is electrically connected to a gate of the first transistor,wherein one of a source and a drain of the sixth transistor iselectrically connected to the gate of the first transistor, wherein agate of the sixth transistor is electrically connected to the gate ofthe second transistor, wherein one of a source and a drain of theseventh transistor is electrically connected to the gate of the firsttransistor, wherein one of a source and a drain of the eighth transistoris electrically connected to the gate of the second transistor, whereina gate of the eighth transistor is electrically connected to the gate ofthe first transistor, wherein one of a source and a drain of the ninthtransistor is electrically connected to the wiring, wherein a gate ofthe ninth transistor is electrically connected to a gate of the fourthtransistor, wherein a first signal is input to the other of the sourceand the drain of the first transistor, wherein a second signal is inputto the gate of the fourth transistor, wherein a potential is supplied tothe other of the source and the drain of the second transistor, whereinthe potential is supplied to the other of the source and the drain ofthe fourth transistor, wherein the potential is supplied to the other ofthe source and the drain of the sixth transistor, wherein the potentialis supplied to the other of the source and the drain of the eighthtransistor, and wherein an output signal is output from the wiring. 12.The display device according to claim 11, wherein the one of the sourceand the drain of the fifth transistor is directly connected to the gateof the first transistor.
 13. The display device according to claim 11,wherein the one of the source and the drain of the sixth transistor isdirectly connected to the gate of the first transistor.
 14. The displaydevice according to claim 11, wherein the one of the source and thedrain of the seventh transistor is directly connected to the gate of thefirst transistor.
 15. The display device according to claim 11, whereinthe gate of the eighth transistor is directly connected to the gate ofthe first transistor.
 16. The display device according to claim 11,wherein the potential is supplied to the other of the source and thedrain of the ninth transistor.